in wow, blizzard uses a start/stop concept for their movements... so,
they get the 'starting moving forward' on W down... and when they
receive W up, they would stop.

if you want shift+WASD to move your alts, but WASD to be blacklisted... you would indeed end up with issues once in a while, depending on how you play. this is why:

you hold down shift... then W.. then left go shift.. then let go W.

you are expecting your alts to move for a moment .. then stop. but they keep moving. why?

this is what passed to the alts:

shift down
shift+W down
shift up

and that is it... since you have W blacklisted, W up never gets sent. this is what you specified by using the black list.

do you have an alternative?