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"Tamaso ma jyotir gamaya" is a famous example. There are innumerable negative mentions of "Tamasic puranas".

Are there scriptures that say Tamas isn't a negative quality?

Ramakrishna has a parable that says that in order to achieve liberation, one must transcend all three gunas.

The Samkhya Darshana has provided the best explanation about the 3 Gunas according to me. It is also one of the ancient scriptures to talk about these Gunas (it is the 1st among the 6 Darshanas).

From SAmkhya kArikA 13:

Sattvam laghu prakAshakamishtamupashtambhakam chalancha rajah |

Guruvaranakameva tamah pradipavaccArthato vrittih ||

Sattva is light (laghu), it reveals (prakAshak) and it is desired (ishta); Rajah is the initiator (pravartak),
and fickle (chanchala); Tamah is heavy (guru) and concealing in nature. These three
Gunas, like a lamp, conjointly work towards achieving the end.

Unlike other scriptures, here Tamah is not described as bad. Here it is mentioned, as a complementary Guna, without which even Rajah and Sattva will be ineffective.

The analogy used is that of the lamp which burns and gives light.

Here fire represents the Sattva, the wick Rajah and the oil is Tamah. It's only when all these 3 combine and work jointly the lamp is able to light up the place. Same is the case with the 3 Gunas. They work together for achieving a particular end.

According to Kashmiri Shaivism ultimate reality has gyaan (cognitive apprehension), kriya (activity) and iccha (will, as vedas say 'May I become many'). When the ultimate reality coagulate in association with maayaa owing to its own will, polarization happens. When it happens gyaan or knowledge (cognitive apprehension) gets reduced to buddhi (intelligence), kriyaa (activity) to ahamkaar (I-hood) and iccha (will) to manas (mind). Now buddhi is out of sattva, ahamkaar is out of tamas and manas out of iccha. This is the reason mind is always wondering, ahamkar or I-hood is very immovable or almost static. This has been commented by mystic Kshemaraj, disciple of mystic Abhinavgupta in the Shiva-Sutra

...Cit or universal consciousness during the course of manifestation
becomes reduced to Citta which consists of buddhi, manas,
and ahamkara. The citta becomes conditioned by its desire
for the pleasure of the objects of sense. The constituent of
buddhi is primarily sattva. that of manas is rajas and that of
aharhkara is tamas. It is this citta which is anu. This citta or
anu is called atma in this context. Using buddhi, manas and
aharhkara it moves about (atati) from one form of existence to
another. Citta is anu or atma i.e. the individual self in this
context... verse 3.1

So when tamas is inseparable to very existence then how can it be invariable connoted negatively? No guna is either superior or inferior, It is just a matter of their role. Tamas means inertia, the ultimate reality is of course immovable or tamsik but when tamas happens to be present in psyche then it implies stagnation or lethargy to change...

I don't know if it has positive connotation anywhere.

Yes, all gunas are considered bad and one has to transcend all gunas.

सत्त्वं सुखे सञ्जयति रजः कर्मणि भारत। ज्ञानमावृत्य तु तमः प्रमादे
सञ्जयत्युत।।14.9।।

14.9 O scion of the Bharata dynasty, sattva attaches one to happiness, rajas to action, while tamas, covering up knowledge, leads to
inadvertence also.

Sattva guna is considered better than other two because it provides
one wisdom (through which one can transcend)

सत्त्वात्सञ्जायते ज्ञानं रजसो लोभ एव च।

प्रमादमोहौ तमसो भवतोऽज्ञानमेव च।।14.17।।

14.17 From Sattva arises knowledge, and greed from Rajas; heedlessness and delusion arise from Tamas, and also ignorance.

18th chapter of gita also explains the same.

सर्वभूतेषु येनैकं भावमव्ययमीक्षते। अविभक्तं विभक्तेषु तज्ज्ञानं विद्धि
सात्त्विकम्

18.20 Know that knowledge to be originating from sattva through which one sees a single, undecaying, undivided Entity in all the diversified
things.

पृथक्त्वेन तु यज्ज्ञानं नानाभावान्पृथग्विधान्। वेत्ति सर्वेषु भूतेषु
तज्ज्ञानं विद्धि राजसम्।।18.21।।

18.21 The knowledge which thinks of the manifold existence in all beings as separate - that comes from Passion.

यत्तु कृत्स्नवदेकस्मिन्कार्ये सक्तमहैतुकम्। अतत्त्वार्थवदल्पं च
तत्तामसमुदाहृतम्।।18.22।।

18.22 But that (knowledge) is said to be born of tamas which is confined to one form as though it were all, which is irrational, not
concern with truth and trivial.

Hope it helps!!

Is“Tamas” always something negative?

Tamas is always bad. It is only good insofar as the Tamasic Puranas turn you away from Tamas.

From the Manusmriti:

12.26 - ‘Sattva’ has been declared to be Knowledge, ‘Tamas,’ to be Ignorance, and ‘Rajas,’ to be Love and Hate;—such is the nature of these, all-pervading and interpenetrating all beings.

How can ignorance ever be good? There is a reason why Shankaracharya in his Advaita system says Vishnu, who is Saguna Brahman, has a Shuddha Sattva Upadhi, and why Ramanujacharya says Vishnu has a Shuddha Sattva body. Also, the residents of Vaikunta also have Shuddha Sattva bodies.

The Manusmriti describes more qualities of Tamas, let's see if they are good in any way:

12.29 - What is mixed with stupefaction, undiscernible, of the nature of sensual objects, incapable of being reasoned about and uncognisable,—one should recognise as ‘Tamas.’

12.33 - Avarice, drowsiness, irresolution, cruelty, disbelief, bad character, habit of begging, and inattentiveness are the characteristics of the quality of ‘Tamas.’

These don't sound like good qualities do they? Some more:

12.35 - When, having done, or doing, or going to do a certain act, a man happen to feel ashamed,—every such act should be understood by the learned to be characterised by the quality of ‘Tamas.’

12.38 - Pleasure is the distinguishing feature of ‘Tamas,’ ‘Wealth’ is described to be that of ‘Rajas,’ and ‘Spiritual Merit’ is the distinguishing feature of ‘Sattva,’—each succeeding one of these being superior to the preceding.

12.40 - Those partaking of ‘Sattva’ reach the state of the gods, those endowed with ‘Rajas,’ the state of men, and those characterised by ‘Tamas,’ the state of beasts; such is the threefold migratory state.

12.42-44 - Inanimate beings, worms, insects, fishes, snakes, tortoise, cattle and wild animals,—represent the lowest state due to the quality of ‘Tamas.’—(42)

Elephants, horses, despised Śūdras, Mlecchas, lions, tigers and boars—represent the middling state due to the quality of ‘Tamas.’—(43)

Cāraṇas, Suparṇas, hypocritical men, Rākṣasas, and Piśācas—represent the highest state among those partaking of the quality of ‘Tamas.’—(44)

Your next question:

Are there scriptures that say Tamas isn't a negative quality?

The Shiva and Shakta Puranas may say that Tamas is a good quality since they are Tamasic or mixed-quality Puranas.

The Sankhya Karika verse Rickross quoted above:

Sattva is light (laghu), it reveals (prakAshak) and it is desired
(ishta); Rajah is the initiator (pravartak), and fickle (chanchala);
Tamah is heavy (guru) and concealing in nature. These three Gunas,
like a lamp, conjointly work towards achieving the end.

by the context presented, may simply mean that one who experiences Tamas will be so disgusted by it, that he will move on to Rajas, and then Sattva. As per Vyasa, the Tamasic Puranas help a Tamasic person turn away from Tamas, and the Rajasic Puranas help a Rajasic person turn away from Rajas, and the Sattvik Puranas help a Sattvic person achieve liberation.