Currently on skyblock the lowest you can stand is Y=1.00, so if you have any non-full block at Y=0 you can't stand on that (e.g. not event a farmland). If only the threshold was a few blocks lower (Y=-5??) we could even fly below the world and build much more easily.
p.s. Building the island low is beneficial because there is less (none) light-updates when you punch a hole through the floor (so no lag spikes), and there are much more natural mob spawns.