To do something blindly is to do it without looking. This word also refers to doing things thoughtlessly. If you blindly accept the friendship ring someone gives you, you might find yourself going to the prom with the wrong date.
There are two related senses of blindly, but both involve being in the dark. If you enter a room blindly, you have your eyes closed or can't find the light switch. A quarterback might blindly throw the ball if he's under pressure and can't see the receivers. Similarly, to make a decision blindly is to do it without careful consideration. Actions taken blindly are done without looking at the risks. People tend to get in trouble when they act blindly.