If Temporary Leblanc dies and I have no cards in hand, can I use Leblanc legend ability?

No. The Deceiver legend ability only triggers on Conquer or Hold — a unit dying, even a Temporary LeBlanc copy, does not satisfy that trigger condition.

Even if you were conquering or holding at the same time, you'd still need at least one card in hand to pay the discard cost. Per Rule 422.3, discard listed as a cost must be completable to activate the ability, and earlier FAQ rulings (FAQ #10237, #8686, #10107) are clear: with zero cards in hand, you cannot activate Deceiver at all.