In Pokemon Diamond Is there a way to learn Counter to Blissey I know he can't learn Seismic Toss and I got that from Leaf Green but can it learn Counter on Diamond?

Yes, there actually is a way for Blissey to know counter. First, you must have a Male Breloom (I know for a fact that Blissey is only female) that knows counter. You must then breed the Blissey with the Breloom. The Chansey (or Happiny) will know counter as an egg move (this usually works, otherwise tell me so)