When installing a lawn it only makes sense to use a blend of different grasses for several reasons. Bluegrass has a germination period of 28 days. That's a long time. Most weed varieties have a germination period of just a few days. Tens of thousands if not millions.
Rye grasses germinate in six days.
Fescues take a little longer, but certainly not 28 days. By using a blend of all three you get grass in less than week. The first plants to come up act as a nurse crop for the slower growing varieties, providing shade and protection for the small seeds. Once all the varieties have germinated you have a very diversified lawn that will stand up against the old and new pests that attack lawns. Lawn pests are selective, a pest that can destroy one variety of grass may not effect the others at all.