Strong first-quarter sales profitability gains were reported yesterday by a diverse group of industrial, consumer goods, utility and insurance companies in the Washington region.

Martin Marietta Corp., of Bethesda, said solid aerospace operations contributed to a record quarter, while Black & Decker Manufacturing posted a 17 precent increase in profits. Potomac Electric Power Co. said earnings improved substantially from last year, Avenco Corp. listed a 72 percent increase in profits and Smithfield Foods said earnings were more than double the 1979 level.

Even though the recent gains were strong, officers of the various companies generally expressed great caution about the balance of 1980. For example, Martin Marietta Chairman J. Donald Rauth said that in "the developing economic recession, our aluminum, chemicals, cement and aggregates businesses will be affected eventually in varying degrees."

Martin earned $34.5 million ($1.39 a share) in the recent quarter, compared with $32.8 million ($1.29) a year earlier as sales rose to $567 million from $437 million. While aerospace earnings rose substantially, aluminum and chemicals earnings were unchanged from last year and the cement and construction aggregates divisions posted lower earnings because of a marked slowdown in homebuilding and some other important segments of the construction industry they serve," Ruth added.

Black & Decker, a Maryland manufacturer of tools, said earnings in the January-March quarter rose to $27.2 million (64 cents a share) compared with $23.2 million (56 cents) a year ago as sales jumped 30 percent to $390 million. In the six months ended March 30, earnings were $53.3 million ($1.27) on sales of $781 million compared with profits of $45.5 million ($1.09) on sales of $605 million.

Chairman Francis Lucier said unit sales rose 19 percent in the quarter with price increases adding 8 percent and favorable foreign currency transaction effect adding 3 percent.

Potomac Electric Power earned $17.16 million (32 cents a share) in the first quarter compared with $12.53 million (23 cents) a year earlier and $15.02 million (31 cents) in the compariable 1978 period. Revenues declined to $172 million from $174 million. c

Directors approved a quarterly dividend of 35 cents a share on common stock, payable June 30 to owners of record May 29. Chairman W. Reid Thompson said the recent earnings remain below levels permitted by area regulatory agencies. He criticized the D.C. public service commission for not yet having acted on a $48 million rate increase proposal submitted in February 1979. D.C. rates now are based on 1977 costs, he stated.

Avemco Corp., a national aviation insurance firm in Bethesda, reported first-quarter earnings of $1.06 million (43 cents a share) compared with $630,000 (25 cents) a year eariler, the previous volume jumped 43 percent to $7.1 million in the quarter.

Smithfield Foods Inc., a Virginia meat packing firm, reported first-quarter profits of $836,000 (38 cents a share) vs. $316,000 (13 cents) as sales fell to usually depressed in the 1979 period, Chairman Joseph Luter III noted. He said profitability from fresh mean operations improved substantially.