Many other have already given good advice, but i would suggest an middle/light grey on the screen to give the fingers around the phone a natural lighting from the screen, that helps sell the shot much better than just a black screen and the too dark contact of finger on the screen in the recording. This is more important in a dark setting, maybe not so much in a brightly sun lit situation like you show here, then the relative small amount of light the screen gives wouldn't be so noticeable.
Avoid tracking markers on screens, with Mocha they usually is not needed, and you have to spend time removing them. :-)
In the case you show with the sun backlight i would have tried to put a white screen on the phone to see if it would show at all in that lighting, and if not, i would put on markers in this one case to just get an easier track and just fake the reflection
...and i would again probably use Mocha to track a mask for the finger... Mocha is really a nice tool
btw. i love this list when so many contribute with good answers to questions. :-)