Why is this a thread? You would need to post your full team for people to see what would really benefit you the most. You could probably study Smogon's list for some basic sets. A Tank is actually a pokemon that is able to switch into something and take minimal damage, but also be able to do considerable damage back. Blissey therefore, is not a tank. You won't be sweeping or KO'ing anything 1 on 1 with a Blissey. Pokemon like Torterra and Regirock are Tanks. You could probably post your team in S&M with some ideas. Please make sure it isn't in-game though, or else we won't want to help you...=P
~T_S