What does this look like?

Had some watering problems. Probably have some lock out due to salt build up on one plant. But what is being locked out I wonder? .

But also... the other seven plants have the same dark purple/brownish thing going on like the upside down leaf. Never seen that before. But first round with cobs. I'm only 43 days in to bloom.

Calcium?

I had similar symptoms and it was due to a ph lockout. I use tap water that i found was ph'd after nutes but plain water was 8 and threw the whole thing off.