I just took seven high-quality scans from Eberhard Rössler's excellent
The U-boat: The Evolution and Technical History of German Submarines. Unfortunately and though the book is packed with diagrams and schematics, there doesn't seem to be a single IXB blueprint in the whole thing.
I have images of IXA and IXD frame lines; detailed deck plans, inboard profile, and sections for the IXD1, IXD2, and IXD1 transport conversion; and a IXC end view that also shows the schnorkel arrangement. There is one more, very large diagram for IXD2 deck plans and inboard profile that I have not yet scanned because it's too big for my scanner. I could scan it at my university's library if you want it.
Would these be useful to you? If so, I can send them your way. I also seem to remember seeing large foldout U-boat blueprints in the university library while doing other U-boat research. I could check to see if there is a IXB there if you want.