I was thinking more on the lines of nutrient burn and over-watering...
I've found this piece and it mentions the spots, not just the tell tale signs of the leaf tips. ;
Leaf Color:
Edges Appear Brown or Burnt
Dark or Purple Leaves
Black or Gray Patches on Leaves
Brown or Dark Spots
If you click onto the pic, it will take you straight to the site and you can check it out for yourself.
Hope these are of help,best luck hope you get it sorted...