Looking for something to keep a blender bottle cool
 

I'm after something that would use something like the gel versions of the ice bricks but only big enough for the bottle and whatever cools it, not a whole ice chest--something that can be stuck in a daypack.

Try first aid/therapy section. They have them in small sizes and as medical device holds the cold/hot longer.

I know this is not what you asked for but I think I have one of these that has never been used. I can always mail it to if you want it.
http://www.blenderbottle.com/blender...ed-sleeve.html

I saw that but it appears to only be insulation. I also don't care about the strap, I'm looking for something to throw in a daypack.

How about something like this? You can cut them up to whatever size you want.

I keep a few of these in my golf bag to keep my drinks and suncreens from cooking in the sun in the summer. My mom cuts some up to fit in her cosmetics bag in the summer so they don't melt or go bad. They don't last as long as the larger ice packs but I like how they are flexible and you can use them with bags that you already own.

If you are checking luggage, these can be reused/recharged by tossing them in the ice-bucket in your room overnight with some ice from the ice machine.

That looks useful but some insulation would also be needed.