I do not write Snape/Black, I play it, but the question as how to get them together stays the same in every genre. And actually, there are a lot of reasons why they could come together. The classical "It was just unreqited lust/love in their schooltimes, and now they realize it" and the just as classical "We are trapped/forced together by circumstances/persons and now must work it out" are just two. One of the main points that is between them, is the fact that they know each other close enough to hate since (at least!) their first schoolyear. I don't know if you have ever actively hated someone for 7 years, but it requires a deep comittment to the enmity and the enemy. You get to know your enemy quite good, you build up familiarity (even if its a negative one).
Sirius has gone through hell. (Rather literally, as many believe you are forced in hell to relive your most terrible moments over and over again. But I disgress.) When he comes out, the world has turned on without him and is now really busy with Voldemort. The last persons left, that are really familiar to him, are Remus and Severus. Even Harry only seems so close and familiar because he reminds Sirius of James. Now, between Remus and Snape, there's not much choice. And between Severus and Sirius, there's one thing lacking: disappointement. The disappointment of betrayal and distrust. Snape is still the same Snape Black knew before Azkaban. Can there be anything more comforting? It may sound perverse, but even hate can be comforting when it reminds you of a better past. This is another, valuable reason. Not something easy to write, but very interesting.

Now, before I really start to babble about my OTP: Always helpful in bringing them together is the use of subconscience and instinct and the playing with their (emotional and physical) needs.