Do you have to BLC to be laterally promoted to Corporal?

When laterally promoting those in my charge, I never considered it in the calculus of my decision making, nor did I have any issues with my S1. My main concern was competence and leadership abilities

You must be otherwise eligible for promotion consideration (not including BLC), and moved to an E5 position.
This is can happen prior to BLC, but as it is the commander's discretion, they can add BLC as a requirement.

Not doctrinally, but the Commander can set that as a criteria if they choose.