Famitsu has posted the results of the color poll for BBCF! For those that don't know, each character in BBCF is getting a handful of colors from past games that were selected by vote. The results are posted on Famitsu.
It is not necessary to get black balls to enhance BRO. Regular balls can enhance it. The fastest way for that is just set then BRO.
If you want the fastest way to get a black ball:
Set > CR or SA
QV > CR or SA
Holding 2 ball then releasing is easier to charge but slightly slower
YRCing can give a lot of black ball possibilities
But because you want black ball into BRO. I'll say just a regular set or QV then CR or SA
However, the opponents pressure is situational and matchup dependent. You don't need to directly make a sequence. You can move or wait then complete