Do you have any other Rooster? The onlyway that this chick is from a BCM rooster is if he has Welsummer heritage(not surprising as both are dark colored eggers and they are used interchangeably to enhanced color), it does not need to be a recent ancestry as wildtype e+ allele can hide for many generations on a bird, the same goes for yellow colored skin(can hid under white for generations), This rooster(if the only rooster on your coop) is clearly ER/e+ W+/w and was crossed to the Legbar hen which is e+/e+ w/w, the chances of the chick being e+/e+, w/w are 50% so quite highI