Well, if it took you a year to fill the prompt, then it's okay that it's taken me almost three weeks to get back and leave a more thorough comment, right...?? But seriously, I had to come back sooner or later, 'cause this ficlet is just perfect! ::flail::

Snape's characterization is just spot-on here; his thoughts are just so perfectly him. The bitterness, the expectation that nothing good can happen for him, and the self-loathing...

The interaction between him and Sirius is so right as well. snarking even when they're not really snarking. Just because they can, just because it's habit, even though neither of them takes it seriously anymore. Because it's something familiar and comfortable, and they need that where they are, and anything else would be strange and frightening.

“Did you hear me, you old bat? Harry’s close!”

“Close to what? Discovering a way to tame that infernal mop of his?”

“No, you bleeding wanker.” Black poked a bony finger into Severus’ shoulder. “Close to clearing our names.”

“Your name, I believe,” Severus said. Potter would have no reason to attempt immunity on Severus’ behalf. Most days, Severus believed Potter had saved him merely so he could watch Severus rot away in prison. It wasn’t as if Severus deserved any less.

There. The auto-snark, and Snape's belief that nothing good can happen to him and that he deserves the bad things anyway. Also, still the refusal to understand Harry. Heh.

“You think I would leave you in here, you tosser? Who would I take pleasure in annoying every moment of the day? Who would I call a great bloody old git, if your ugly snout wasn’t around.” Black curled his fingers around Severus’ wrist and Severus told himself he was too cold to pull away; it would require too much effort. “I told him, both of us, or not at all. But it’s Harry of course.”

Black rested his head against the stone wall and closed his eyes. “He always does the right thing. Not like us.”

Abjlyvabuk!! Affection expressed perfectly for these two! Again, snark and insults as endearments because they're comfortable and safe, where "typical" expressions of affection would be weird and scary. Everything with this pair is in what they don't say, and HOW they say what they do say.

Also, Snape lying to himself for his own peace of mind (even though he doesn't really believe himself)... And the admission that they're similar in a lot of ways... and maybe even that they've been wrong in the way they treated each other in the past...

Just a whole world of FLAIL! I can't believe how well you captured the dynamic in 545 words! <333333