Blinds that are not well brushed in and backed against something with a dark outline can be noticed by deer really easily. Also, it could have been a bit of bad luck - deer was looking that way right as you turned your head or something. It helps a lot if you can leave a blind in an area for long periods for them to get accustomed, but that's not always possible obviously.
Ground blinds and archery equipment can be tough. The best recipe for success that I've seen is to set up in an area where your range of shots requires little or no body shifting. If you are set up in an area with a wide field of view, you're probably too exposed and/or likely to get busted while rotating your body. I recommend extra effort on making your face unrecognizable - a camo net has been the most effective for me (had staring contests with does under 30 yards). The downside is that visibility can be hindered. Be sure to practice shooting with the covering to make sure it's possible.
