I would go in and prove this, but I don't have a copy of BLHack (for good reasons).
It isn't.
It's more or less a by-product of the float hack
The invisibility issue happens when a client lags on a vehicle, I've noticed this when my clients in my server lagged because of my internet and from other users just lagging themselves too.
So, he may or may not have had it, who knows.
But as for an invisibility hack in BLHack, it's not included.
does anyone have the link to the blhack fix?
There isn't exactly a fix for blhack but for the speedhack, Ipquarx posted the source here.
http://forum.blockland.us/index.php?topic=271003.0PM him for a compiled version if you can't compile from the source yourself.