It struck me as a watched this again that it is kind of the same story. Both are about how the ghosts of past hunt us all and the affect a strange, exotic place can have on us all.
- No animal was hurt during the making of this burger -
The two films have been compared before. They both are in other worldy locations. Both as you say, have characters haunted by the past. In both films, the characters thought they had put the past to rest but then it comes flooding back again. Ironic, given that they are as far away from the actual incidents as possible. In Black Narcissus, one goes bezerk with very tragic consequences. A similar situation happens in Solyaris with a character's self- termination. Both seem to wrap up with "what are we doing here."
Let's not forget the last minutes which clearly reminded me of "Alien the 8th Passenger"! you know? these fast shots of Ruth's silhouette escaping. Then subjective shots from her as she spies on sister Clodagh. My favourite part of the movie by the way!