Most people abandon a sleep mask because of pressure on the eyelids rather than because it let light in.
Contoured versus flat
A flat mask rests directly on the eyes. A contoured one has moulded cups so there is space in front of the eyelids. If you feel your lashes brushing fabric, or you dislike pressure on closed eyes, contoured is the shape to try.
Light leaks at the nose
The gap beside the nose is where light gets in on almost every mask. A nose wing or an adjustable bridge is what closes it. This matters more than the darkness of the fabric.
The strap
A single flat adjustable strap sits better than elastic that digs in, particularly for side sleepers whose head is against a pillow. Check the buckle does not sit at the back of the head where it will be lain on.
Fabric and washing
A mask is against your face every night. Whether it can be washed easily is a practical consideration, not a minor one.