Please don't be deflated - it's a beautiful painting of a very beautiful animal. As you are new to ArtRage and it's tools, you have done a commendable job with this painting. I look forward to seeing more from you.
EDIT: I found the picture you used as a reference and your painting is very close to the reference photo - including the 'blackness' - perhaps blend the area where the foreleg attaches to the chest - there seems to be a division/line that doesn't appear in the photo and wouldn't be there in real life. I think the Friesian stallion's neck looks a little overdeveloped because we can't see his thick wavy mane. A little more work on the mane would improve that part of the painting. You captured the likeness very well.
BTW - here is a link to the photographer's site -
http://hypofocus.com/index.html You asked about copyright in another post. As the photographer sells stock photos of horses and has also published a book of her horse photos - this photo would be copyright protected - you could contact her and ask permission to use her photos as references for paintings.