On Pokemon black, if the foe uses toxic spikes, and you switch out to a Pokemon holding the AIR BALLOON, will toxic spikes take effect?
No, Toxic Spikes (nor regular Spikes) will not take effect. It works the same as if you have Levitate or are Flying type.
No, not until the Balloon pops. The user will not take damage from spikes when it drops to the gorund from the Balloon pooping, but the next time it switches in it will.