I think that the find statement here is the one with the implicit wait, so Capybara will wait until the element is on the page, but won’t wait for it to become visible.
Here, you would want Capybara to wait for the visible element to appear, which should be achievable by specifying the visible option:
expect(page).to have_selector('#blah', visible: true)
I haven’t tried it, but the ignore_hidden_elements configuration option might be useful here as well, if you wanted find to always wait for visible elements.