How are these stitches invisible? (Photo)

This may be the eyelid all cleaned up immediately after surgery but before the placement of sutures. It may be the eyelid after the removal of sutures such as a subcuticular stitch. The important thing to understand is that the closure of the blepharoplasty is important but not as important as the design and execution of the surgery including what precisely was perform internally in the eyelid. In my opinion the blepharoplasty represented in this photograph will only be mediocre at best. The crease is too high, the eyelid platform skin is lax so the eyelid will heal with lash ptosis and likely crepey eyelid platform skin. It is far too early to tell but these are early hall marks of what I call post upper blepharoplasty syndrome.

Blepharoplasty scars are usually placed in the natural upper eyelid crease. Scars are pink for the first six months or more as the wound heals. Longer scars outside the eyelid crease are performed if excess upper eyelid skin; a lateral brow lift may minimize the scar length.
The photo you are showing looks like a very early postop with swelling and still has purple marking pens applied; it looks like a running intracuticular closure with internal dissolving sutures.