Answering this myself, since I just saw a different thread with the solution :
Go to : System Preferences -> Security & Privacy -> In the Privacy tab, chose "Screen Recording". On this screen make sure that the Skitch checkbox is checked.
Im running Catalina (10.15.3) and Skitch (2.8.1 (264719)).
Whenever I try to do a Screen Snap (Command Shift 5) whatever I try to capture, end up becoming a black box in the Skitch app.