So if Blacks are disproportionately arrested and convicted for pot possession/trafficking, am I being dense or something if I assume then that legalization will be a 'pro-black' measure? 

Or is this just another opportunity to use the words "systemic racism"?

Actually, I think if you are poor you are more likely to end up in jail as:

a) You can't afford an expensive lawyer
b) You likely don't have that "nice" family background that earns you brownie points with the judge
c) People with money usually find non-violent ways to steal and 'White Collar' crime isn't taken very seriously in this country
d) And if you can get what you want by legally bribing politicians, why would you need to resort to illegal actions?

And in all of this, I haven't had to invoke 'systemic racism' once.