A Bluegrass lawn needs plenty of water to grow well. In drought it will discolor, thin, and eventually die. In general, a Bluegrass lawn will need 1-1.5 inches of water every 7-10 days during the summer, whether from rain or irrigation. Try to water deeply and infrequently. For example, apply 1 inch of water on a single day rather than .25 inches each day for four days.
If you have an automatic sprinkler system, take a moment to investigate your control box and settings. Do not be shy about increasing the pre-set cycles during droughts or turning the system off entirely after heavy rainfall. Your lawn will lose some of its dark green color in a drought, but it might die if swamped in poorly drained soil.
If your lawn with a manual sprinkler, place a rain gauge or straight-sided jar in the lawn to measure the amount of water applied. Try not to apply more, or less, than an inch at a time.