Potomac Electric Power Co. yesterday reported a 46 percent decline in its profit for the first six months of the year, blaming the drop mostly on new electric rate designs that change the way its revenue and earnings are distributed throughout the year.

The Washington-based utility, which provides electricity to about 1.8 million area residents, said its second-quarter profit fell 22 percent, in part due to mild weather in June.

Pepco's first-half earnings totaled $36.2 million (32 cents a share), compared with $66.6 million (66 cents) in the 1989 first half.

Revenue fell 4 percent in the half, to $595.3 million from $621.5 million a year earlier.

In the second quarter, Pepco earned $30.1 million (28 cents), compared with $38.8 million (39 cents) in the 1989 second period.

Revenue fell 2 percent in the quarter, to $312.1 million from $318.7 million.

Besides the new rate design and the weather's effect, Pepco said its payments for the final 150 megawatts of capacity purchased from Ohio Edison have not been reflected in District electric rates.

The D.C. Public Service Commission Tuesday approved new rates that will reflect those costs, and they are expected to take effect in early August, Pepco said.

Black & Decker Corp., the Towson, Md.-based tool and appliance manufacturer, yesterday reported a $16.1 million (26 cent) second-quarter profit, compared with a $5.1 million loss in the same period a year earlier.

Revenue soared 44 percent in the quarter, to $1.2 billion from $847.6 million.

The results included revenue and earnings of two subsidiaries -- Information and Electronic Systems and Dynapert -- that had been reported on the consolidated balance sheet as businesses held for sale.

The companies contributed about $220 million in revenue during the quarter.

For the first half, Black & Decker said, it earned $26.1 million (43 cents), up 26 percent from $20.7 million (35 cents) in the 1989 first half.

Revenue for the half rose 57 percent, to $2.2 billion from $1.42 billion.