Well, for a start, Bird Cherry (Prunus padus) fruits later than the wild cherry or Gean (Prunus avium). Gean fruits are variable, we have them through from yellowish with a red blush, through to red & deep purple/black. Don't forget that some you come across may be deliberate plantings or garden escapes too.
Bird cherries are indeed black, but are held along a branch like sting or strand whereas wild cherries hang in disinct bunches on long stems eminating from a single point. Have a look at the link for a good set of pics that give you a comparison of flowers & fruit.
http://www.hainaultforest.co.uk/5Bird cherry.htm
I have eaten Bird cherries and found some to be ok - but that's a matter of trial and error - as with wild ones some may be sweeter than others.I've also cooked with them, sieving out the stones.