Why is my tv screen blue everytime I turn on the TV?

The idea is for your AT&T U-verse remote to send commands to both the STB and the TV to turn on.  If your STB doesn't get a signal to turn on, then you'll get the STB standby screen (which is blue) and you'll need to press the OK button on your AT&T remote to wake up the STB.

This could be because you're turning the TV on with the TV's power button or the TV's remote instead of the AT&T remote.

If you're using the AT&T remote to turn your TV on, then it could be because your remote is programmed to only send the TV power on.  I'd point you at the remote programming instructions:

Using your U-verse TV & remote