Thanks for the warning, well taken. I've used Blacken-It on white metal with no ill effects, and I'll ask some local friends if they had any experience with britannia metal specifically. If worse comes to worse, I'll test it out on a block.
I'm glad to hear about the longitivity of your blocks; I understand its best to use diluted white glue for rigging issues for the same reason.
Thanks for the insight,
Jim