[crazy voyager]

I'd say it's a foul when B1 makes contacts with A1's legs (he's in his cylinder and out of defencive position) but you could also have a double foul, B1 for blocking and A1 for elbowing

but afterall it's a judgment call, but I'd say call the Defensive foul first, if elbows still makes contact, make it a double personal foul (if you deam it as an intentional elbow, unsportmanslike)

that's how I think I would call it anyway (and that's fiba=no violation for swinging elbows)