Note: Perfect block teams were nerfed in a recent patch. You can no longer create a team that takes 0 damage when blocking in Contest of Champions. The game creators felt it was unfair to players who didn't have the characters required to build those teams.
"Perfect block" teams are unfortunately somewhat of a misnomer in Marvel's Contest of Champions, due to the fact that there's a Perfect Block Mastery that has nothing to do with creating a perfect block team. The stat you want to pay attention to is Block Proficiency. Specifically, you want to make sure that your team has at least a 43% Block Proficiency in order to take zero damage while blocking.
To do this, you want to do two things:
- Maximize your Block Proficiency Mastery (under Defense) to 4/4 for an 8% increase.
- Put together teams that have the Leadership Synergy bonus, which gives an additional boost to Block Proficiency.
For example, when I pair up my 4-star Dr. Strange with 4-star Scarlet Witch, I get a 20% increase to Block Proficiency.
If I also add my 3-star Dr. Strange to this team, I get an additional 15% Block Proficiency, for a total of
20% + 15% + 8% = 43%
I'm already at my goal with only three characters. Since the Synergy Bonus screen tells us that All Champions gain the block proficiency, I can add any other two champions I want to round out my team. All of them will have "perfect block."