Why does my red wine reduction go 'grainy'?
I have to wonder if you are using a wooden utensil to 'scrape the meat remnants'.  If so, stop using wood.  The wood will char and leave bits in your sauce, creating the bitterness.  Try slowing down the process a bit and using less heat, at least initially as you deglaze the pan. The graininess could be charred bits of food which are burned before they are loosened from the pan surface; this could account for the black spots too.

Cooking is love, and love doesn't hurry.

:)