Couple points
Firstly, it seems to me that any caster worth their salt should be able to target the center of a Darkness with Dispel Magic from outside the Darkness. Hm, or maybe people were talking about casters
inside the Darkness...
If this tactic is upsetting the game, the DM needs to think about how to handle it. There are quite a few ways:
1) Negate the tactic. This is what I do -- IMC, Magic of Faerun is banned, so the spell never comes up. If the DM just plain doesn't like this kind of thing happening, this is what to do.
2) Nerf the tactic. As several others have suggested, make a house rule so that the spell, or the blindsight ability, doesn't work in a way that enables this tactic.
3) Ignore the tactic. This, presumably, is what's happening right now -- the DM isn't doing anything in particular about the tactic, and it's causing problems.
4) Embrace the tactic. This is the interesting solution.
If these spells exist and are common, and the tactic really is that effective, then this shouldn't be a "PC tactic" -- it should be a world-wide tactic. As such, countermeasures would be developed. I would start thinking about things that any creature could use to negate this tactic. Some might require creating new spells or items: an alchemical potion that, when its flash is broken, creates daylight for 1 minute; a
Higher Daylight spell that is identical to Deeper Darkness except that it creates light. The DM should come up with a bunch of these, assume that people use them, and adjust encounters accordingly.
Alternatively, simplying upping the EL of encounters, because the party is that much more powerful, makes sense.
As an aside, using this tactic back against the PCs is silly; they are prepared for it. The DM should come up with /other/ effective strategies.
Finally, remember that the purpose of D&D is entertainment. If no one is having fun because of this tactic, and the DM doesn't want to do all the work required to revise his world and adventures to account for it, then no one should complain if the DM says "this combo doesn't work any more". Think of it as switching Darrens or something.
