Can i react hidden cards on battlefields i control during my beginning phase? the battlefield is dusk rose lab with my hidden card "Hidden Blade" with one temporary unit.

Yes — you can play Hidden Blade from facedown at Dusk Rose Lab during your own Beginning Phase. Your temporary unit's [Temporary] trigger ("Kill me at the start of your Beginning Phase") is mandatory, so it goes on the chain as the phase starts; that creates a Closed State and therefore an ordinary Reaction window, and a card that's been Hidden gains Reaction while facedown (Rule 811.6). So you reveal and play Hidden Blade for [0] in response, it resolves first (LIFO), then the Temporary trigger resolves.

Two things to keep in mind. First, the card must have been hidden on a previous turn — you can't play it the turn you hid it (Rule 811.1.b). Second, because it's played from facedown, Hidden Blade's target must be a unit at Dusk Rose Lab (Rule 811.1.d.2), so you can't use this window to kill something at another battlefield — killing your own token with it does draw you 2 off the "its controller draws 2" clause.

One caution: Dusk Rose Lab's own ability is a "you may," so it only joins the chain if you opt in; if you use both, the Lab's kill is a cost paid at finalization, so you can't sacrifice the same unit to the Lab and also target it with Hidden Blade.