I just ran across your note through some google searches I am doing on Maryland orphanage records.
I realize this reply is years from when you first wrote this note, but I thought this might help...
My aunts and uncles spent time at St. Elizabeth's Home for Colored Infants and Children in the early 1900's. I did find them in ANCESTRY records. They were census records, they were recorded as 'inmates' at the St Elizabeth's Home. So, ancestry has census records listing all of the children at the home in a particular year.
This may help you. Maybe ancestry has St Mary's Orphanage records as well.
Or -- maybe you were able to locate the orphanage records through some other source. If you were, I would love to hear how you found out more.