Both. I have put the blind out in advance and then the same day. If you find a good food source where the crows are feeding in early morning, then that's the place to be at first light. It hasn't seemed to bother them that the blind is there, even if I set it up the same morning.
One morning I found crows feeding at an area that's basically a dump. Someone had thrown out a deer carcas and the crows had found it. Naturally, I scared them away when I set up the blind (which didn't take 5 minutes) but they returned in about an hour and I got two shots.
Farming areas, especially where cattle are, are good places. The farmers will feed the cattle grain or hay, and crows tend to congregate in those places. I guess the hay has seeds in it, and then too, the cattle's hooves turn up the soil, exposing seeds and insects. If there is a lone tree near a barn, that's a good place since the sentry will generally sit in the tree while the others feed.
I have also set up the blind overlooking decoys and called with the Foxpro. Took a crow that way too.
This year is the first time I have tried the blind, and as I mentioned before, the main purpose was to get video and to conceal my movement with the camera etc.
Places like this are great for crow sniping. Notice the tree near the barn.
Here are two crows I shot with a .22 short HP, from a Browning auto. It's the gallery
model and only shoots shorts. Got these behind my house.