surprised you didn't vice n hammer it,far harder with a blade
Haven't got a vice.
I use a pair of plumbers grips to hold the IHS and then push on the corners till the seal breaks on one of them.
Once the black stuff is broken on one corner it is easy to cut through the rest with a blade.
Trying to break into the black glue with a blade is real hard, I did that before and it puts your chip in real danger of scratching.