Those do not look like burns to me, they look like chafe/compression points where the spare was sucked up against the frame when the spare was raised into position. Maybe the mechanism raising the spare was over-torqued, I don't know. But if those were burns, I suspect the thin white lettering outlines would be black, not still white. The bluish tint is probably a bit of paint/coating that was on the undercarriage before the spare was sucked up against the frame and it simply transferred to the tire due to compression & vibration action as the truck was driven normally, or perhaps it is the mold-release agent on the tire itself, and it disappeared form the rest of the tire due to exposure to the elements underneath the truck, and the only place the mold-release agent remains is at the compression points with the frame.