How do you get a Blissey on Pokemon Diamond?
you get it by getting 2 big hearts on a chansey and lvling it up
you check the hearts with the happiness checker app
if you mean the egg that makes happiny evolve, no. happiny didnt exist when fire red came out. plus, its impossible to find a wild blissey in the wild. the only way to get it to hold the egg is to migrate it to Pokemon diamond,pearl,platinum,heart gold, or soul silver and then give it to blissey after you catch it in pal park.
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)