A diss review might not be very helpful, it can usually be spotted a mile off as a feelgood exercise for some would-be snark. But a thoughtful critical review that makes a valid comment on the state of things that be is gold.
Saying "that's not art" is always a mistake, as history amply proves. But saying "this could be more interesting" or "what's up with this particular circle-jerk moment" can be.
Seriously, if art blogs have one problem to focus it on it will not be that they are too critical. More so than other media outlets, blogs are written partly out of the desire to be respected and possibly liked by the community you blog for. Critical writing is rare enough, saying it's counter-productive is, well, counter-productive. Without criticism we are left with a Pinterest-style world.
Just don't ever say you don't like my work, ok? And keep your reviews friendly enough for the lite yoghurt demographic, they got dollars to spend.