[adrianmonk]

My preferred alternative to electrical tape is a piece of aluminum foil stuck on with painter's masking tape. Aluminum foil (or any metal) is really excellent at blocking all the light, and painter's tape doesn't leave residue.

[beatgammit]

If you're not worried about residue, you can get aluminum foil tape, which is quite handy for other household repairs as well.

[edoceo]

Gaff tape.

[toomuchtodo]

Gaffers tape is the holy grail IMHO. So many good uses for it.