Blissey is outclassed by it's pre evolution chansey, due to the Eviolite. Blissey, however, has the advantage of being able to hold an item.
Blissey is an amazing defensive Pokemon, sporting the highest HP stat of any Pokemon and a very high special defense. It has an amazing support movepool, including reliable recovery, heal bell, wish and toxic just to name a few.
It's special attack is ok, so you can run flamethrower if you want to burn physical attackers that would normally give blissey troube. However, seismic toss is usually prefered for consistent damage.
In a nutshell, Blissey is a great choice for your team.
