Black & Decker Manufacturing Co. of Towson, Md., experienced a gain of 51 percent in net income during the quarter ended Sept. 30 and an increase of 116 percent for the fiscal year.

Quarterly earnings totaled $24.5 million (48 cents per share) on revenue of $482.9 million, compared with $16.2 million (36 cents) on sales of $278.5 million. The increase reflects the acquisition of housewares operations from General Electric Co., the company said.

For the year, net income totaled $95.4 million ($1.95) on sales of $1.5 billion versus $44.2 million (65 cents) on $1.2 billion in sales a year earlier when restructuring costs amounted to 70 cents per share.

Smithfield Foods Inc. reported that profits increased by 198 percent in the quarter ended Oct. 28, driven by significant improvements in margins, according to Chairman Joseph W. Luter III. The Arlington company had net income of $1.6 million (52 cents per share), compared with $521,000 (19 cents) in the same quarter last year. Revenue was $146.3 million, compared with $133.9 million during the same period in 1983.

Six months results showed profits of $820,000 (33 cents) on sales of $277.7 million, versus profits of $1.3 million (43 cents) on sales of $270.1 million the previous year.

The Rouse Co. of Columbia, Md., best known for downtown developments such as Baltimore's Harbor Place, reported third-quarter net earnings of $31.7 million ($2.09 per share) on revenue and sales of $102 million. That figure includes the $50 million sale of Rouse Real Estate Finance in September to Paine Webber, which netted $30.2 million. Income from continuing operations amounted to $1.4 million (9 cents) versus $1.9 million (13 cents) the previous year, down 29 percent.

For the first nine months, net income totaled $38.7 million ($2.55 per share) on revenue and sales of $197.3 million, compared with earnings of $7.7 million (51 cents) on revenue of $125 million in the same period last year. Income from continuing operations in 1984 was $7 million (46 cents) versus $5.7 million (38 cents), up 23 percent.

B. F. Saul Real Estate Investment Trust suffered a net loss of $2.7 million (47 cents a share) in the quarter that ended Sept. 30 and a loss for the fiscal year of $5.1 million (87 cents). That contrasts with a profit of $902,000 (15 cents) for the same quarter in 1983 and an $8.2 million profit ($1.37) for the fiscal year. Gross revenue increased slightly in both periods, amounting to $19.9 million at quarter's end and $67.9 million at year-end.

The downturn was primarily the result of increased interest and debt expense, the firm said. The yearly loss included an $843,000 expense when the REIT refinanced some mortgages. The previous year's profit was attributed in part to a $10 million gain on property sales plus a $4 million tax loss carryover.

Primark Corp. reported a third-quarter loss of $6.7 million (71 cents a share), compared with a loss of $7.7 million (87 cents) for the same period last year.

The McLean holding company, which owns Michigan Consolidated Gas Co., said the results were typical for the third quarter because of the seasonal nature of the gas utility business.

Revenue for the quarter was $270,712, a slight decrease compared with $284,699 the year before.

Earnings for the nine months ended Sept. 30 were $38,233 ($4.07), up 178 percent from $13,678 ($1.54) the year before. Revenue for the period was $1.6 million this year, compared with $1.3 million in 1983.