I've heard that the Ioun Wyrd is blind, except for its blindsight, is this true? It seems much more powerful then the other familiars on most points so I'm wondering if there is any downside to it?
In fact, besides its blindsight (not blindsense), the Ioun Wyrd is of the construct type and all constructs have darkvision and low-light vision. Nothing in its monster entry indicates exceptions to this general rule. It can't speak, however.
There is no additional fluff in the Familiar Folio except for mentions of Nex and Thuvia edited out for OGL reasons that might justify this belief.
To create one you need CL5 and spend 1500 gp plus an ioun stone in its creation, so it's more expensive than say the banana slug. The ability to fly and blindsense is cool, somewhat offset by its inability to speak tactically/scouting-wise. And +1 to the caster's natural armor is a good benefit. Its ability to store ioun stones is not (I've never said "boy it would be a good tactical idea to strap 50,000 gp worth of loot to my familiar").