If you fail BLC do you have to wait 6 months to enroll in the course? Or can you enroll before that time frame?

First, you will have to prove to your BN and BDE CSM that you should be sent back to BLC if you fail the course. I've known some CSM that would make you wait a year before you could try again after failing the course.

You cannot return for six months.
You can enroll for within that timeframe for a report date that is more than six months from the failed course.

https://www.ssi.army.mil/ncoa/NCOAPolicies/Policy_Memo_36.pdf

While there are policies that support a second opportunity, it is not the rule, rather the exception that it is approved. While there are no gimmies in Military programs they are built to ensure success by all. If one fails they have missed multiple opportunities to recover whatever the weakness may be.

Then one has to ask themselves; why should the CSM's in the unit, and at the Academy use up another slot on the same individual when there are many in the wings awaiting an opportunity.

For me it would require a very strong endorsement from the Academy leadership along with the COC before it would even be considered.

Nah, you don't have to wait the full 6 months to give it another shot. You can re-enroll earlier, but it might depend on your specific situation or the school's policies.