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.