Depends what blow up means...
I won't name old ones, but new rappers that have the opportunity to blow up (cross over success)
Vic Mensa
GoldLink
In straight Hip Hop blow up terms...
Al Doe
Kevin Gates
Bibby
Herb
Edit: which brings up another point, why don't we ever discuss new music here. It's always after an artist gets huge that we get threads discussing them.