I always pictured it concave-edge sharp, mostly because that's how it looks to be portrayed in all of the official art. And Andalites do often use their tails for chopping off limbs and heads, so that would make sense.
Also, at first, I thought you were asking about the convex-concave orientation of the blade, whether it's concave- or convex-edge towards the tip of the tail. And the official portrayals of Andalites don't entirely agree about that. Ax and Elfangor (main series covers, The Andalite Chronicles) seem to have tails that are concave-edge towards the tip of the tail. But Alloran and Aldrea (VISSER, The Hork-Bajir Chronicles) have blades that face the opposite direction. So, idk. Concave-tip would seem to be a better orientation for stabbing forward, but concave-down would seem to be better for pulling the blade in a sweeping-slicing motion. Not sure which one would be better overall. Again, though, I tend to picture the one that is more often depicted, that being Ax's concave-tip stabby-tail.