A client-sided mod that enforces the Blindness effect on the player, configurable per world.
A client-sided mod that gives you the Blindness (or Darkness) effect all the time.
Configurable per world and server.

Additionally, BlindMe can be used as a server mod affecting all players.
Licensed under MIT, made by hibi.
By default, new worlds and servers have the Blindness effect applied by default.
To change this, use the settings screen (accessible through Mod Menu), or use the /blindme command.
A world can have the blindness effect disabled, too.