I'm going to silver solder an Enfield magazine box to the floorplate and then rust blue it as a unit- I know that silver doesn't rust, and was wondering if there is any way to blacken the joint-line?

No.
After I caustic blue silver solder, it sort of turns a little yellowy like a nickled finish on a gun.
Personally I think it would look sort of cool but if it is internal you wouldn't see it anyways.

-Spencer

some cold blues will work on silver solder, I think Oxpho blue is one of them.

brownells makes a solder black (blue)

though I haven't been tickled with either one

http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=17662&title=SOLDER+BLACK

jeffe