Prime Minister Pieter W. Botha announced the appointment of a seven-man Cabinet committee today to evolve a policy for South Africa's 10 million urban blacks, whom the government has so far excluded from its plans for constitutional change.

Botha gave no indication what the new policy might look like, how the committee would operate or when it would report. While opposition politicians were skeptical of it producing any major reform, they welcomed the implicit admission that government policy has failed to deal with this most developed sector of the country's majority black population.

In a speech following Botha's announcement in Parliament here, Pieter G. Koornhof, the minister in charge of African affairs, said: "The urban blacks must be recognized. To pretend they do not exist would be irresponsible and dangerous."

The ruling National Party's policy for all of the country's now 21 million blacks has been to give them nominal independence in 10 tribal "homelands" situated in undeveloped rural areas.

Less than half of the blacks live in the "homelands." The rest are either laborers on white-owned farms or the working class of South Africa's rapidly growing industrial cities, where they live in segregated townships. Soweto, outside Johannesburg, has a population estimated at 1.5 million.

In terms of the original theory of apartheid, as the government's segregationist policy is called, the urban blacks were supposed to be a temporary phenomenon, fading away as the "homelands" developed.

The admission of blacks to the townships is strictly controlled, and for years such features of permanency as stores and banks were excluded. The only trading licences permitted were for the "day-to-day necessities," as the government regulations put it--meaning vegetable stalls, milk distributors and the like.

Urban blacks were not allowed to own their homes and could rent them only from the government department administering the townships.

Some of this has been changing as the government has come to realize that the idea of the urban blacks fading away is a pipedream.

Four years ago, Botha recognized their permanency in the townships. Since then, commercial development has been permitted and a 99-year leasehold scheme introduced. But the government has remained adamant that the urban blacks can have political rights only in the "homelands." Today's announcement indicates that change is coming here, too.

How much change is unclear. Botha is introducing a constitution that will give token representation to the Asian and mixed-race minorities through parliamentary chambers separate from that of the whites, but he has said there will not be a fourth chamber for urban blacks.

Local newspapers speculate that the townships will be designated as special voting districts of the "homelands," which would be given representation in a new umbrella assembly.

Chris Heunis, the constitutional affairs minister who will head the new Cabinet committee, told a press conference tonight that the committee would be prepared to consider the question of ownership rights for urban blacks as well.