Are there diminishing returns on crowd control abilities such as Blind?
I was wondering if 4 monks with Blinding Flash could keep mobs permanently stunned, since the skill can be runed to blind for 4 seconds, and the cooldown is 15 seconds
There are two major factors in determining how effective CC is. The first is mob type. Rares and Champion Packs tend to resist the magnitude of CC effects - this is even explicitly stated on Blinding Flash's tooltip (elites get a chance to miss, rather than being blinded). Likewise, bosses are also highly resistant to CC. Diablo, for instance, can basically ignore the snare from a Wizard's Blizzard.
The other factor is difficulty - CC effects last less long on higher difficulties. At Hell and Inferno, stuns, knockbacks, and the like sometimes simply fail to occur in the first place.
So to answer your posed scenario - sure, 4 monks could permanently CC a group of Normal mobs - but a group of mobs in Normal difficulty aren't something you'd need to permanently CC in the first place.