Black & Decker Manufacturing, a large Maryland-based tool maker, yesterday reported decreased earnings for its fiscal year ended Sept. 28 of $90.1 million ($2.14 a share) compared with $94.4 million ($2.25 a share). Revenues were $1.4 billion compared with $1.2 billion for the previous year.

Fourth-quarter net income was $19.6 million (46 cents), down from $29.6 million (70 cents). Revenues for the Towson firm were $325 million, up from $314 million.

President Francis P. Lucier said "we expect our sales growth to progressively improve during 1981 compared with the 7 percent increase in 1980's fourth quarter. This is based upon an expected improvement in economic conditions in the United States and takes into account a continuation of economic pressures in Europe.

Best Products Co. of Ashland, Va., yesterday reported decreased first-quarter earnings blamed in part on the high costs of opening eight new showrooms during that period.Net income was $2.4 million (25 cents a share) compared with $4.1 million (46 cents). Revenues for the quarter ended Oct. 4 were $187 million, up from $154 million.

Andrew M. Lewis, the firm's president, said the earnings decline was caused by "the higher start-up in operating expenses of our eight first-quarter showroom opening compared with no openings in last year's first quarter."

Washington Corp. of Chevy Chase (formerly Capital Mortgage Investments) reported third-quarter earnings of $222,442 (10 cents a share) after an extraordinary gain of $215,748 and a reduction of $189,000. During the third quarter last year the company had a net loss of $28,795.

Nine-month earnings were $1 million (46 cents) compared with $222,449 (10 cents) a year earlier. Revenues were $1.3 million compared with $2.5 million last year.