I hunt west texas, so there aren't any trees tall or straight enough for a ladder/tree stand. I'm considering a 4x4 box bling with tall vertical windows as a possible solution. I'd set the blind roughly 25 yards away from the feeder behind a mesquite tree. Good idea or bad idea?

I think you may wish you'd gone bigger. As for being in the open, so to speak, I hunt in Del Rio area and we don't have trees big enough to hunt out of either. So, we use short tripods or blinds - some elevated and some on the ground. Don't be too concerned with the deer being afraid of your blind, provided you don't give them a reason to be suspicious.

Set it up well before season so they grow accustomed to it being their.
but I agree with hammer, you should really consider making it bigger.
we use either a 6x6 footprint or a 4x8 so we can get as far back in the blind as possible. We also paint the insides all black
Deer can pick up movement very quickly

Is July too late to set the blind up?

We've got the same problem on our farm... No trees. I bowhunt out of a tripod! I tried the ground blind for a few years but I really like being elevated when it comes to bow hunting!

You could always build a platform and just set a pop up blind on top of it

I have a combination blind for sale in the classifieds, based on the redneck predator approx 5x6 hexagonal. We had it up close for bow hunting and it worked great, only problem was the deer at the lease showed up too early or too late to shoot (legal time thingy). Had we hunted exclusively from this blind we would have for sure gotten shots off, but we switched to our gun blinds as the season progressed. The deer didn't know we were up in that blind, it worked really well and I believe I set the blind up around September time last year....