I have checked with my players and no one thought the occational big raises were an issue.
That's important to consider.
I can’t ban dark raises, but I guess I can just ask players to refrain from it.
That's correct, you can. Which effectively bans it. That's also important to understand - it's your game and you can run it any way you please. If you decide that dark raises are bad for the game, make it clear to the players who do it that continuing to do it will get them uninvited. Don't leave it out there as a vague guideline; be straight-up and crystal clear with your players.
Bear in mind that if you do this,
those players might quit.
Sometimes you can't please everyone, and you'll have to decide who will be happy and stay vs. who will be upset and quit. But you should always run the kind of game that
you enjoy playing in.
Which I feel he should have done if he thought they were such a big issue.
He should have, but it's not unreasonable for people to be upset and yet not say anything about it. People aren't perfect machines, not even in Sweden.
Thoughts?
TL;DR
Op hosts 5/5 game with some restrictions.
Two players start raising in the dark.
Op as a result start 3-betting those light.
Another player gets upset and doesn’t wanna play in OP’s game anymore.
The first thing you'll need to do is decide whether you want to continue to allow those two players to continue to raise in the dark.
If not, then have a discussion with them, encourage them to stop, let them know they won't get invited back if they don't stop, and try to persuade them to keep coming to your games and enjoying themselves even without doing blind raises. And be prepared for them to stop coming.
If so, then have a discussion with the other player, encourage him to put up with the blind raises, make sure he knows that it's actually a losing play for the raisers and so is profitable for him as a good player, and try to persuade him to keep coming to your games and enjoying himself even with having to face occasional blind raises. And be prepared for him to stop coming (and probably to stop bringing his other friends).
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If it were me, I'd ask the two players to stop inflating the stakes by blind raising. I'm sensitive to the stakes changing and want to protect my players. But that's me. You should do what feels right to you.