Thank you! reipan came up with the idea of having Blackadder and Castiel as the B/C combination, because she is an evil genius.

(Hmm, yes, that's a bit tricky. Alastair, Bobby, Castiel, Derren, Ellen... erm... erm... no, F is where I run out of characters from your fandoms. (But G could be Grigori!))