Maryland was first recognized as a "Free State" on November 1, 1864.   On that date, the Maryland Constitution of 1864 took effect.   By its provisions, slavery within the State's borders was abolished, and Maryland, indeed, became a free state.

March 1 to June 1, 1860 - In these three months, 136 slaves were manumitted by their owners in Frederick County.   This was in advance of a new state law which took effect on June 1st, 1860, which prohibited the freeing of slaves.
[from John W. Ashbury's "History Moments"]

Note - You can also Search on Main Page on the given name, surname or "Slave, Slaves, negro, negroes, colored, coloured, mulatto" for more listings on the whole MidMdRoots website (except those linked to other sites or in pdfs).