During the winter, many people sow a temporary, cool-season grass like Annual Ryegrass over their dormant warm-season grass lawns. Since Kentucky Bluegrass remains green during the winter, there is no reason to overseed. Thinning Kentucky Bluegrass lawns should be reseeded with Kentucky Bluegrass, not overseeded with ryegrass.
Overseeding with ryegrass is a horrible idea since the ryegrass will compete with your Kentucky Bluegrass for nutrients, water, soil, and sunlight. Once hot weather arrives, the ryegrass will die and leave your Kentucky bluegrass lawn weaker and thinner than before.