I dont know if something like that was created before (haven't found anything), so I'll try to request it:
Ok, so when some player flashes his teammate, he gets blind for X seconds (white screen fade maybe?). If he flashes more than one teammate with his flashbang, then time is doubled/tripled/and so on (like if he flashed one guy, he gets blind for 4 seconds, flashed 2 guys -- blind for 8 seconds, flashed 3 guys -- blind for 12 seconds and so on).