If you find one mite, I would take a proactive approach and assume that all of your other snakes have them. Mites can spread rapidly from cage to cage. I've had them spread to snakes that I have in other rooms, across several feet of carpet, very rapidly. I had great success using provent-a-mite. The good thing about provent-a-mite is that you don't have to change out your substrate, and it can last for a month.
I don't have experience with black knight, but apparently it is very safe for your snake, as well as very effective. Immediate relief was given to my snake by soaking it in 95 degree F water with a couple of drops of liquid soap mixed in.
Mark