I was just wondering if i could feed betta frys blackworms?

Question

and if i can, how do i grow them?

Answer 1

if they are small enough yes

Answer 2

Blackworms are FAR too large-- really, they'll be larger than the fry.  Also, there is a massive risk of introducing pathogens (bacteria and parasites in particular) to the young, undeveloped fry, which is almost a sure death sentence.  Just go with microworms or baby brine shrimp... even crushed flakes would work.