Product review (4 stars): Works great for covering pass-through vents to block light.
Worked great on covering our pass-through vents.  I can't say if it would block the air flow of a HVAC vent though as that wasn't the reason we purchased it.  For us we placed them on the pass-through vents above our door to block the light from the hallway and for that they worked great.  They are flexible enough to cover the raised louvers and to stick around the edges.