That cap needs replacing. The black is a combination of carbon and copper oxide, resistive elements. It’ll “drop” voltage as the spike tries to jump the now-larger and resistive-element-coated gap between the rotor and the plug wire connection, leaving less to jump the arc on the sparkplug.
NOTE: you should always change the rotor along with the cap.
Interestingly, carbon is often called a “resistive conductor”. Its resistance drops when it’s compressed. But that’d just trivia, not relevant to your problem.