Following the wonderful bugs with invisible heroes I think they could add it as a special tourney rule.
Invisible raid- all opponents are invisible, you will not know what you are fighting and will only be shown the defence team upon completion of raid.
Yes it will be pure luck if your colour stack is correct but will add a new challenge
Nice suggestion and only rainbow should be allowed for that one. You’d have to set up for all eventualities and team synergy would be much more important.
Maybe it was good to fade in one random enemy every 2nd turn.
New rule: Blind Battle 
A brilliant idea, voted!
Also, @Grumpigamer, I’ll appreciate if you could change the topic title to better reflect the idea. Something that would help people understand the meaning of your idea just by reading the title. That way your idea will get more recognizable while getting the votes.
You could just tape up your screen 
I’ve always thought that this would be a good experiment for the rigged-boards conspiracy brigade.
Do you think that enemy’s special skills should be hidden too? They may break the conspiracy.
And actually I don’t think that it will always be possible. SG may hide the special skill animation, but effects on your heroes probably should be still visible. For example, you wouldn’t see how our beloved Fisherman casts spell on you, but you will see that all your heroes are de-buffed and sharing the damage now.
P.S. While knowing that the hero exists by recognizing his special skill, you still won’t know his position.
Upd: Now, when I think more about it… Maybe giving some tips about the opponent would be actually great and fun. It could be like solving a puzzle (the game is E & Puzzles, right? 
) - you will understand who you are fighting with, step by step: by seeing special skill animations and effects, by seeing how many damage points you apply to the enemy, by seeing resistance effects (e.g. if the enemy doesn’t burn, you know that it’s probably Kingston).
I already changed the title, but I do not understand here, I think invisible during fight is not a good idea, but invisible before fight is okay for me, so if we bring example green mono, and during fight, all opponent is mono red, surprise 
.
EDIT: I changed again the tittle without “before fight”.
One more thought about the idea:
(and I’m not trying to criticize, I’m just thinking loud about all cons and pros, 'cause I actually like the idea very much)
I see “Blind Battle” more like a war bonus rather than a tournament mode, and I’ll explain why.
If you look on all existing tournament modes, they are being applied to the defense team and to the attack team in the same way. In other way, they change the mechanics without giving advantage to anyone. It isn’t so for the “Blind Battle” mode, 'cause it gives serious advantage to the defense team. So yes, it would be fun, but it would be also very hard. You will need to have an extra strong team and a big amount of luck to survive to the day 3, and that’s what hurts.
War bonuses, in turn, are designed to give advantage to the defense team. That’s why I think that the idea could be applied there. Just need to figure out how to show it correctly.
The idea only came to me due to a friend continually having the visual bug of invisible heroes. He was fighting 2 invisible zombies in a raid today. Just thought it would be a fun challenge
If life gives you lemons, take it as a challenge.
… no, wait.
Another funny take on this would be that your defence & offence teams are randomly selected from your top 20-30 strongest heroes…
Introduce some TRUE RNG
then, there should be another new rules: Gamble Fighter
It can be also new rule with: Trial Quest heroes, so like for example: Time for Survival, so only Barbarian and Rogue restict heroes, etc. But this can be not friendly for new player of course.
You can go the opposite way with the classes… Rather than ONLY these classes, you say any class BUT Rogue and Paladin (example)… Same as the colours
You could also add a rule that your specials fire randomly at any hero except the one you choose.
This. Invisible during the fight is the dumbest thing I could imagine in this game. Obviously, this will never happen, so I’m not worried about…
This could be fun as a War Rule but probably won’t work as a Tourney rule. Its too one-sided for the Tourneys.
A twist that could make it work in the tourney: when selecting your teams, including your Defense, you can see your heroes color ONLY. Name and power are invisible!
Seems a bit like fog of war