Simon Jenkins, in The Guardian online newspaper, writes:
How can the British Tommy, the jolly tar or the air ace confront the dreaded foreigner, when a defence secretary cannot say boo to George Osborne? Be a man, Fox, they cry. The black spot is under the plate. The pistol is in the gents.
I googled for the exact phrase, and the only hit is this article. Googling for "black spot under plate" just gets me a lot of stuff about toenails. What on earth could he mean?