In the 1930s, Aunt Ferebe Rogers talked about her life as a slave. Even though she was more than a hundred years old, she still remembered the dogs:
"When slaves ran away, they always put the bloodhounds on their tracks. Master always kept one hound name Rock. I can hear him now when they were on the track, calling 'Hurrah, Rock, hurrah, Rock! Catch him!'...After a slave was caught, he was brought home and put in chains."
From the Federal Writers' Project Slave Narratives, 1936-38. See site.
Slave owners used dogs to find runaway slaves. The dogs would sniff the ground to find out where the person had gone. 
Bloodhounds are funny and kind dogs. They can find people who are lost. But some slave owners trained them to find slaves.