I thought it looked like some grey brown near the center of the stave. in that last photo, maybe just the pith and it looks bigger/more widespread than it is.
Seeing as how it is lifted in the same spot as the opposite limb, maybe you were stressing those inner limbs in your design, thickness taper. If you are not willing to let the wood take the fall...then I blame you!
Yup, marking your wood is a good practice. So is burnishing. I think Marc St. Luis wrote in a recent Q&A column that he quite burnishing after he looked at the back fibers under a microscope/mag lens before and aft and determined the burnishing was crushing back fibers. I think this caused some healthy re-examination of burnishing. I am now of the opinion that burnishing does crush the outer back fibers, but I think that is the very reason it is an effective practice. Think of the back fibers as layers, and that they are only as strong as the weakest link in the that layer. So if you have a little nick somewhere in that outermost layer of fibers, then you have a chink in your armor. If you burnish the back, then you are crushing the entire outer layer and passing the work onto the next layer below, which is unviolated and now protected by a compressed/crushed outer layer of fibers. Maybe or maybe not that would have helped your bow.
