Hiya.
When I first read the post, I thought..."Ok...so...uh...what's the problem again? Are you upset that the players are being smart and using their brains?".
Then I reread it and read some other posts.
IIRC, there was a statement of "So I ruled on the spot that the opponent was blinded". If that is literally what you did, *that* was your mistake, not letting them try it.
There is absolutely nothing wrong with players wanting to carry a large belt pouch full of flour in hopes of using it to blind their opponent. However, it shouldn't be automatic. Have a simple to hit roll need to be made, and the target gets a save to resist/avoid it. Don't forget, if a PC is trying this, they have to have *at least* one hand (likely two; they have to open the pouch, remember?). This would be "dangerous", to say the least, in combat.
If players come up with clever and effective plans and tactics...GREAT! *That* is one of the points of the game. And,
IMHO, that point trumps rules crap *any* day of the week.
Bottom line: always let your players try reasonable things if they want to. It's NOT the DM's job to say "No" just because he doesn't like something the players are doing...the DM's job is to remain neutral and say "Ok, here's what you have to roll/do...[stuff]...do you still want to try it?". Sure, sometimes there are "No" moments (ex; a fighter decides he wants to unfurl the scroll of Fireball and read it...), but, as I said, "reasonable things" should always be given a chance.
^_^
Paul L. Ming