The prospect of the first black senator from NJ will bring blacks out in droves. Blacks vote 9 to 1 for Democrats around the nation. In NJ, it's probably 100 to 0 for historic elections like this one. Mid term elections have low turnouts. High black turnout combined with low white turnout could lead to a landslide for the Dems. This why Christie was smart to decouple the historic senatorial race - with Booker running - from the general election. And I'm someone who thinks Christie should just switch parties and be done with it.
True that.
I still think that Christie would win, but his margin would be much less. Holding both elections in November would hurt the Republicans in the down-ticket races.
The Democrat machine normally only gets cranked up once every couple of years -- now it has to be cranked up three times. It isn't cheap to drag their core voters out to the voting booth.
Could be wrong, but Booker doesn't seem the kind of black politician who appeals to black racists. Just not radical enough, like he seems to try to be tough on crime in Newark. Also he presents too much of a middle class image. Plus, running statewide, he would have to be careful not to turn off whites. And I don't think he's as much of a machine guy as his black predecessors as mayor in Newark. Remember too he'd be running for Senate, not president, in an off-year election.
All in all, you may be overestimating the potential black turnout effect for Booker's Senate candidacy. There may be some, but nothing like Obama.