If you hit enter on an equation and it stays black, it means an internal error occurred in Andes while processing the equation, so ANDES failed to determine whether the equation was correct or not.
Black equations will not be used by the solve-for tool. However, it is likely that you do not need the equation to solve the problem.
If you encounter this situation, you may wish to email the ANDES team so the bug may be fixed in a future version of ANDES. To be most useful, an error report should include a copy of the solution file including the black equation. To save a copy of the solution file in an accessible place, go to the File menu and select "Save Copy As", then navigate to your Desktop to save a .fbd file where you can easily find it. Then include that .fbd file as an attachment in the bug report.