Highest blues I've got were 298. It's totally random and seems pretty rare. Your current light level determines what items will drop at. Even though I still constantly get like 260-280 blues when I'm at 301 light
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I've heard it also factors on the difficulty of the mission and/or difficulty of the specific enemy that dropped it. I've seen it go both ways, like the 297 AR that I got from patrolling the cosmodrome, and 260s from Kings Fall engrams.