I'm not used to Python code (which is all that black touches) as being notably slow to build, nor am I used to incremental build systems for Python byte compilation.

And I expect in a project with 2 developers which is big enough for things to be slow, then most of the files will be unchanged semantically speaking, only swapping back and forth between two syntactically re-blackened representations, so wouldn't an caching build system be able to cache both forms?

(NB: I said "caching build system" because an incremental build system which expects time linear order, wouldn't be that helpful in bisection, which jumps back-and-forth through the commits.)