Reading on this page makes me inclined to believe that it's required to have 2x 3N9XX mezzanine adapters in the fabric b/c slots. Is this correct? If so that's likely my issue since they are blank on all 4 blades.[URL]
I recently got a VRTX and acquired 4 M640 Blades for it. All 4 are reporting UEFI0068: the mezzanine card configuration used is not supported on this chassis. The only card installed are the dual 10gb nic in fabric 1 Node1: JVFVRNode 2: XWKGYNode 3: XWKGYNode 4: XWKGY The switch in the vrtx is the 1gb switch. 
Dell VRTX Mezzanine Mismatch