I have always thought that it was an American slang term for the plain black guns of English makers. This is most often seen in Purdey and H&H guns.
Considering that the finish on the actions had to be perfect and free from any flaw that engraving would hide, it is understandable that much care went into their being built.
I have also heard that they were a tribute to Queen Victoria's death, but don't put much stock in that.
Best,
John
PS---I don't think of the guns as the Black Diamond Tobin as a funeral gun.
