Hi;
Apply a threshold to test each pixel (ie 10/255). If pixel value is smaller than threshold, increase counter.
But in this way probably you can count also some black pixels around the eyelash (any other black random or single pixels).
So you can do some testings using neighbor pixels.
ie if pixel next, above and below to tested pixel, is also below the defined threshold then accept it as black.
I hope you got my point.
Good luck.