This is probably more info than you need or want, but here goes.
The importer of Blake products is Doug Moody at Saint Brendan's Isle( SBI). He has bolts on hand for the Blake at $5.95 each. SBI is well known in the cruising community for its excellent mail service
. We used them for years.
Sail has it right, the most common metal, used by Spartan, Perko, Buck, Groco and Apollo is leaded
red brass. That's the term they use along with 85-5-5-5 (% copper-tin-zinc-lead). It's also called composite bronze and Copper Alloy 836.
I believe that the Blake is a specially treated form of red brass (85% copper and 15% zinc.). My understanding is that the metal is super heated in a manner that creates a resistant barrier. However, I don't know how that effects nobility and galvanic action, especially when the different metals are used like in your case.
Confused, I was, but those are the facts.
Bottom line, go with the general consensus and use bronze.
Also, Hamilton Marine
is a great place and knowledgeable people, but with Jamestown Distributors just up the street, I'd ask their advice and run with it.
It's amazing how much and how little we know about one of the most important components of our boats.