Volvo Penta D1-30 and 130 Saildrive Issue

[Flying Scotsman]

Hi I have a starboard D1-30 Volvo with a 130 sail drive. I got caught at low tide and stuck a little on a sand bank but was able to get clear and continued on my journey for about 2 hours. We motor sailed most of the way in light winds and everything seemed fine till I tried to set the anchor. When selecting reverse the starboard engine coughed and spluttered and if I tried to rev it I got black smoke and some black substance coming from the exhaust. If I tried to select reverse after that it would pretty much just conk out. I messed about with it and found that in neutral it revs fine and also in forward it behaves fine but as soon as I select reverse it either dies or struggles and won't rev. It was revving hard and working when I had to get off the sand bank so I think it may have been the cause but not one hundred per cent sure. Any help would be appreciated.