Please explain

The sequence of events for the outer planets is actually a little different from what you described. First, U was discovered, but scientists realized that their initial assumptions did not match the observed orbit, so they made new assumptions, and those assumptions were found to be correct. In this case, the negative evidence or black swan was the fact that the orbit did not match the expected orbit. A black swan should be something that disproves a theory. This was not what Uranus did, rather, it showed that an initial assumption was incorrect. We do however have a true black swan that served to disprove a theory. The details of mercurys orbit ran counter to newtons theory of gravity, and helped to eventually disconfirm that theory and confirm the relativity theory of gravity.