Release the buttons once the screen goes black and the Samsung logo appears.
An over-the-air (OTA) update may have crashed, forcing the phone into Download Mode for a manual fix. blue download screen on samsung
Water exposure or internal shorts can sometimes trigger the bootloader to enter Odin Mode automatically. What if the Screen Says "Do Not Turn Off Target"? Release the buttons once the screen goes black
Look closely at the text on the blue screen. Some versions of the Download Mode display a small menu. blue download screen on samsung