Blindness fog does not fade out smoothly

As the Blindness effect is wearing off, the fog retreats in fits and starts instead of continuously. This isn't very perceptible in normal gameplay but it's easy to see at low tick rates; see the attached video (the very abrupt change at the end is MC-248194, not this). Darkness fog does not have this issue (although the Darkness pulsing effect does have a similar issue, which I'll report separately) so copying some code from Darkness over to Blindness should fix the issue.