IMO...
don't feed when the lights are out if you ever want to see them.
frozen blood worms will float (at least all the cubes I have gotten)
I always just thaw them in a cup with water, and pour it into the tank. The pouring action forces the worms off the top and 1/2 way down the tank. The knifefish will learn to come to the food faster.
If the tetras eat it all, just feed more at the same time so they can't possibly get it all.