I think it's important to note that the blue fibers that were found to be "consistent with cotton towel material" were not the only fibers found. We also know that JB was allegedly wiped down with something and that the item in question was never found.
According to Dr. Henry Lee's notes on the fiber evidence collected, he noted black cotton, blue and brown, and black wool along with "other" wool. These were from the thigh swabs. Black and blue cotton were found in the pubic region, also noted black, blue and brown fibers. And red polyester (I think that's the word he wrote, it starts with "p o l y e" and the last bit is hard to read).
I would say from looking at the picture of the PJ's that BR was wearing on Christmas morning (12/25), I would describe that blue color more as "royal blue" as opposed to dark. Police described some fibers (as reported by the Daily Camera) as "Navy blue pilings or fuzz balls". The color was a closer match to JR's robe which was found on the floor of his office. However that was apparently ruled out as the source.
It is certainly of interest that BR's PJ's were apparently never found and turned over to police. That was not the only article of Ramsey clothing that went "missing". And the Ramseys never actually accepted the findings of the autopsy report. JR's reaction to it being pointed out in court by Bruce Levin that fibers found on the body were a match to the Israeli shirt he was wearing the night of the murder are particularly telling IMO.