Yes, you can Ready an action for when the target Blinks back
The rules on taking the Ready action state that (emphasis mine):
First, you decide what perceivable circumstance will trigger your reaction.
The villain being affected by the Blink spell returns at the start of his turn:
At the start of your next turn, and when the spell ends if you are on the Ethereal Plane, you return to an unoccupied space of your choice that you can see within 10 feet of the space you vanished from.
Someone suddenly popping out of the Ethereal Plane to the current plane certainly counts as a perceivable circumstance so this is a perfectly valid strategy to try to counter the use of the Blink spell.
However the Blink spell still provides some defensive advantages:
- By using the ready action, characters are forced to expend their reactions which means that their reactions are unavailable for the rest of the villain's turn
- The villain can choose a place they can see within 10 ft of where they vanished to return, allowing them to return in a more advantageous position (such as behind cover or simply out of range of melee-focused characters1)
- Some features such as Extra Attack only function on your own turn so a PC that readies the Attack action will only be able to do a single attack against the villain
1. Suggested by Admiral Jota