I have a crkbd but never heard of Blackpill, is that one of the ones that’s already defined in qmk? If not you need to create the mapping from layout to pins, I have no idea how that is done though.
If it’s defined have you flashed both halves? Before using it? I imagine so otherwise the right side wouldn’t work at all, but just in case.
At least for crkbd the keymap is defined as if it was a single thing, so you don’t need to specify split keyboard, the mapping of pins to keys should take care of that, but the crkbd is always split so that might be the reason you need to define that in your keyboard.
I know it’s a bit inconvenient, but have you tried setting it to not split and flash the right hand size with a MASTER_RIGHT config defined?