I seem to have gotten around what I believe is a limitation with mocks and block passing by doing something like this:

mock(Klass).run.with_any_args do |*args|
block_that_was_passed = args.last
block_that_was_passed.should eq some_verifcation_string
end

If there's a more idiomatic way, love to see it, otherwise I guess this is closed.

No, there isn't a built-in way to do this. I don't know if I plan on adding support for this any time soon, but it's an interesting use case. The workaround you came up with looks like the best solution so far (although you need to say block.call and not simply block).

It's also curious to me why it doesn't work like this instead:

mock(Klass).run.with_any_args do |*args, &block|
  block.should eq some_verification_string
end

I'll go ahead and keep this issue open because I want to look into that in particular.