It's hard to go wrong with electrical tape. I haven't found an LED that could shine through it yet.

Electrical tape was useless on my ASUS router with its blinding blue LEDs. Three layers didn't even get rid of the problem, because the lights are so bright that they shine through the white plastic casing of the router. It's incredibly annoying. I finally placed the router inside a cereal box.

Electrical tape turns to goop after a few months(years), black duct tape lasts slightly longer

Same here. Black electrical tape. Also useful on laptop/monitor/tablet/phone webcams and mics. :)