I keep bitches apart from dogs for the entire 3 weeks. Some bitches ovulate very early in the heat cycle. Others ovulate late. Usually the darker blood turns lighter around the time of ovulation. Usually dogs will change how they relate to her. At first they are just grooming her and checking her out. Then they will try to mount her. At some point, she will stand and flag her tail. That is called standing heat. Usually there is a three day window.
All bitches are different.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.