If it is the opponent's turn and I use a hidden hostile takeover to takeover their attacking unit on the battlefield, can I on this same chain use a hidden blade to kill the unit I took over once it becomes my possession?

Yes, you can do this.

According to the RiftJudge FAQ #8808, you can use Hidden Blade on a unit you have taken control of with Hostile Takeover to kill it and draw 2 cards.

Sequence of Events

1. Hostile Takeover is played from "Hidden" as a reaction to the opponent's attack.
2. Hostile Takeover resolves: You gain control of the enemy unit and ready it.
3. Because the chain is still open (or you have priority in the showdown), you can now play your Hidden Blade from "Hidden" at that same battlefield.
4. Hidden Blade targets the unit you now control.
5. Hidden Blade resolves: The unit is killed. Because you are the controller of the unit at the time of resolution, you draw 2 cards.

Important Considerations

• Targeting Restriction: As per RiftJudge FAQ #8911, you can only play a card from "Hidden" to target units at the battlefield where that card was hidden. Ensure both your Hostile Takeover and Hidden Blade are hidden at the same battlefield where the unit is located.
• Chain Timing: You must have priority to play the Hidden Blade after Hostile Takeover resolves. Since you are in a showdown, you can add the Hidden Blade to the chain as a reaction after the Hostile Takeover resolves, provided you have not passed priority.
• Unit Presence: The unit must remain on the battlefield until Hidden Blade resolves. If the unit is removed from the battlefield before Hidden Blade resolves, the target becomes illegal and the effect will fail.