You can book shore excursions (including Blue Lagoon) at different times depending on factors, but they all require that you have your cruise paid in full first.
If you are a first time Disney cruiser, you can book excursions 75 days out for Carribean and Bahamas cruises
If you are Silver level (1-5 cruises), you can book them 90 days out
If you are Gold level (6-10 cruises), you can book them 105 days out
If you are Platinum level (11+ cruise), you can book them 120 days out
If you try to book directly with Blue Lagoon and tell them you're on
DCL, they will tell you to book through DCL. Some have reported not revealing that fact and getting in, but DCL buys the majority of spots when they are in port (at least as far as I've read on certain posts).
The payment is charged to your stateroom.