In general when they say "upper body invincibility" what it actually means is their hitbox only covers their legs. This does not make them 100% immune to all airborne attacks - if the hitboxes connect, you still get hit. As for Dust, the only reason it would hit you while blocking is if you accidentally stopped blocking too early, or switched to low block. All standing Dust attacks are overheads, even if they don't look like it.
I don't know what character you play, so I'll just show you a character I know. This is Potemkin's 6P.
If you have an attack that reaches that hitbox at his legs, you'll still hit, no matter where it came from. Greed Sever reaches low enough that a poorly-timed 6P will still get hit.
Check this board for #R netplay stuff. For BBCTPC you can try this board but I doubt you'll get much interest.