Can you continue a keto diet after a BBL? Or can I lose weight after a BBL without losing any volume in my butt?

Hi. If you lose weight the volume will decrease, remember you have fat cells (adyposites) that enlarge or diminish depending on your fat deposits. They will continue their behaviour wherever they are transfered.International Member of the American Society of Plastic Surgeons (ASPS)
Member of the International Society of Aesthetic Plastic Surgery (ISAPS)

A BBL works by transferring the fat cells to the buttock.  If somebody looses weight after a BBL, fat cells everywhere in the body will shrink.  Thus the fat distribution will reflex the changes after the BBL.  So the answer is yes, you will lose volume, but the volume loss should be everywhere you have fat and not only in the buttock.
I am unaware of any evidence that shows that a ketogenic diet would alter fat distribution differently than other diets of similar caloric intake.  Best of luck and for specific advice please seek an in-person consultation.