No it wouldn't. Some white people would be helped, but if drug laws are enforced 5-10 times more against blacks than whites and drug convictions and sentencing are also much higher for blacks than for whites, decriminalizing drugs would clearly help blacks a lot more than whites. As was pointed out, we know for a fact those laws to some extent were put in place specifically to hurt blacks, so getting rid of them would be a black issue.
Honestly, the truth is there isn't much policy out there that could be pushed that would help blacks specifically, unless its something like no income tax for blacks or free secondary education for blacks. I think you really have to get Congress on board for that type of thing though, so you can't really pin a failure to accomplish that solely on the president.