You'll have to post a pic if you want specific advice. You would also do best to learn more & not ditch your farrier based just on anabel's post(no offense anabel!
) There are a variety of things it could be - due to long toes, jamming or other imbalance, could be pure & simply a bash that has caused a bruise here & there. If the 'bruises' are in rings it could also be a sign of laminitis, be that from metabolic or mechanical cause.
Oh and white feet are no weaker than dark(which are only pigmented on the outer layer anyway), but I think the story came about because as you have noticed, you see things in them that you can't see in dark hooves.