How to make a random eye blink expression in after effects?

There are a couple of ways you can do this. One is to create a comp that contains your blinking eye. Nest This inside the comp that has your character. Apply time remapping to the nested eye comp. Then you can simply add keyframes where you want the eyes to blink. If you don’t want Values to interpolate between frames, Select the key frames, right click on them, and choose toggle hold keyframes. This is the best way to create blinks In specific places. If you want to randomise this. You could add an expression to time remapping. There is the wiggle expression or for truly random values, check out the random expression. Here is the course on expressions. It’s worth learning more about them as they can help you refer to times dimensions et cetera. Hope this helps.

Learning After Effects Expressions

I found a handful of threads covering this. The expressions tend to be pretty complicated.
CreativeCOW

Eye-Blink Expression : Adobe After Effects Expressions

Another alternative, if you haven't looked at it, is using Character Animator. It's super-easy to get realistic blinks in there - because you can set it to just be using your actual blinks. Depending on how much of your character is already built, you could look at doing the facial animation in there (or even just the eyes), then bringing it into AE to attach to a body, if you've already done some of that work.