Postestrous / Metestrous Hemorrhage.
Normal vaginal discharge of blood tinged mucus, typically observed 48 hours after the onset of estrus in 40% of cycling cows and 60% of cycling heifers. The amount of blood varies from faint streaks of blood to profuse bleeding. The color varies from that of fresh red blood, to dark, black and subsequently chocolate brown as the interval from the onset of estrus to its observance increases.
Drost M (1970)