When selecting "Blacken Borders" filter, one has to put in the number of pixels of which the left, right, upper or bottom borders shall be set to black. If a user doesn't have the correct number at hand, he needs to find that out using some indirect way - e. g. using the "Crop" filter as a test where the interface allows to put in data as left, right, upper or bottom as well but simultaneously shows the content of the currently active frame so one can do test inputs and immediately can see the effect on the output.
The user can take the result from Crop filter testing and use that as input value for the Blacken Borders filter, however this means a couple of steps more to do than it could be.
Even better of course would be a graphic input facility where one could drag the mouse of a border over the frame content until it fits the user's purpose.
I'm using ADM 2.6.12 om Windows 7 or Windows 10 respectively.