Ok, so here's an update.....
I managed to do as I was advised and boot into text mode (CTRL+ALT+F1). I had no idea what to do from there, but I managed to do that much!!
I also considered reinstalling (or removing and installing again). I have no idea how to remove Mint (bearing in mind I am dual booting with Windows7), but found I could just remove the partition.
Questions: Firstly, could my black screen be due to not enough space allocated for the Mint OS? If so, can I resize the partition without corrupting the OS?
Secondly, if none of the above is relevant, can I just remove the partition, therefore removing GIMP also, then reinstall my Windows MBR and try again? My Windows7 install, although not containing any of my important files (which are stored on an external), would be a very labourious reinstall should anything go wrong!!
Many thanx again for any information provided.