It’s called Lena Blackburne Baseball Rubbing mud, and is used to allow pitchers to have a better grip over the balls. Before this mud came along, baseballs were rubbed in water and dirt, but this usually gave the ball a brown appearance.
Sometimes tobacco juice, shoe polish, and dirt were also used. When Lena Blackburne was a third base coach for the Philadelphia Athletics, he decided to set off on a search for something better to use, all the way back in 1938.
He found the perfect mixture of mud within 10 years, said to be somewhere near Palmyra, New Jersey, and then founded the company to sell it. By the 1950’s, it was so popular that every major league team was using it.
Today, only the company’s owner, Jim Bintliff, knows the location. The mud is cleaned and screened, and a secret ingredient is added before sale (so don’t get your hopes up about finding some laying on the ground). Bintliff takes 1,000 pounds of mud once a year, every year, and sells it the next season. It’s today considered the perfect rubbing mud.