It's obviously not completely irrelevant, as your argument depends on a causal link between black support for crack sentencing laws and the adoption of those laws. In fact, no such causality is established, and it is indeed unlikely, given that the laws were passed by an overwhelmingly white legislature in the grip of a nationwide (largely unjustified) freakout about crime.

John Pfaff's "Locked In" is a great source for the background on this stuff (Pfaff would be the first to say that drug sentencing laws are not the reason for mass incarceration in the US, but that's not the question at issue in this thread).

And white politicions supported harsher sentences in the past as well. Do they speak for us today? Did they speak for everyone back then? Last I checked you only needed a simple majority to get elected.