Acrobat DC toolbutton appears as black square until you minimize/restore the Application window
Use 'Open' to add an Enfocus PitStop toolset. It appears in the right-side ribbon with the 'Active' icon. Click on this icon => it changes into a black square.
When using 'Add' to add the same toolset, it appears using the 'Inactive' icon, clicking it makes it Active but it is not showing the 'Active' icon.
Bert Van Kerckhove commented
Tested with Acrobat DC 2019.010.20091, Windows 7, Enfocus PitStop 2018