Might also be possible because under the right conditions at least one blue colorant I know of (cupric oxide) can also produce oxblood. But be that as it may . . .
I've wondered a lot about these too. Seems like such a natural combination of colors, somehow. Perhaps there were a few happy accidents. Looks like there were at least two, with yours and the piece Brian found!
I have long had a pale blue marble that looks like a lightly striped striped opaque, with the striping looking suspiciously like oxblood. I keep it with my tiny collection of CACs. In my more rational moments I think "Surely [don't call me Shirley] that's a mahogany brown, or something, and not oxblood . . ." or even "Well it's not exactly right, but I suppose it could be one of those Euro types . . ."
I don't know why I've never compared the blue with the blue of my one cherished Persian turquoise, bought from Alan back in the day. Probably because I keep them in separate boxes, at least 6 feet from each other. Oh well. Marble homework, oh shucks.
Can't wait to see what might turn up here. Would love to see more . . .