Can you play Hidden Blade from Hidden on your temporary unit on its temporary trigger on your beginning phase to draw 2?

Yes, you can — but only if Hidden Blade is already hidden (facedown) at a battlefield you control.

Hidden Blade from hand lacks Reaction, so you cannot play it to respond to the Temporary trigger going onto the chain at the start of your Beginning Phase.

However, if Hidden Blade is already hidden at a battlefield (Rule 811.6 — facedown Hidden cards gain Reaction), you can flip and play it at reaction speed to kill your Temporary unit before the Temporary trigger resolves. You'll draw 2 cards as its controller.

This is a direct application of FAQ #5313: hidden cards can be played at reaction speed to anything on the chain, including phase-change triggers like Temporary.