I do not believe there is a Capybara::RSpecMatchers for what you want. However, you could use the underlying Capybara::Node::Matchers.
Try:
find(".blah").has_no_text?.should be_true
If you really want to use the Capybara::RSpecMatchers, you could use have_content
with a regex that looks for any non-whitespace character.
find(".blah").should have_content(/[^\s]/)