Maryland's black population increased by about 37 percent during the 1970s, the state planning department said yesterday. The agency said that the 957,418 blacks counted in the 1980 census represent almost 23 percent of Maryland's population. The total number of blacks in 1970 was 698,388, nearly 18 percent of the population.

The black population increase in Maryland was greater than the national increase of about 17 percent, the report said. Michel Lettre, an analyst with the department, attributed most of the increase to black migration into the state, most of it coming in the movement of blacks from Washington to the Maryland suburbs, particularly Prince George's County.

Without the flow into Prince George's, the state-wide increase would have been 16.5 percent, comparable to the national figure. In Prince George's, the 247,888 blacks counted in the 1980 census represent a 171 percent increase during the decade. Blacks in that county now account for 37 percent of its population.

Baltimore became a majority black city during the 1970s. In the early part of the decade, blacks accounted for 46 percent of the city's population. By 1980, that percentage had increased to nearly 55 percent, or 430,934 people.