IT IS CLEAR that the Supreme Court dealt a sharp blow to black political aspirations in the South last week when it refused to break up the system that almost guarantees Mobile, Ala., an all-white local government. But it is not at all clear that what the justices did was, from the legal point of view, wrong or even that their decision represented a serious setback to civil rights.

Stripped of its nuances, the issue the court had to resolve was whether Mobile should be forced to abandon the form of local government it has had since 1911 because that particular arrangement makes the election of black officials almost imposible. The answer -- that Mobile can stay the way it is -- derails the legal theory that civil rights lawyers had hoped would force a shift from at-large elections to ward or district elections in cities all over the country.

In this case, the theory asserted that the existance of the commission system of government in Mobile unconstitutionally dilutes the votes of blacks. Because the commission system mandates at-large elections and because Mobile is still given to racial bloc voting, no black has ever been elected to the city government and none is likely to be in the foreseeable future. If the city, which is 35 percent black, were broken into wards, the election of some blacks would be almost assured.

The trouble with this thesis, in the view of several members of the court, is that it rests on the assumption that the constitutionality of any law depends upon the effects it has on minority groups. They think laws should be judged on the intent with which they were written. This argument -- whether it is effect or intent -- has been going on among the justices for years, primarily because neither standard is really satisfactory. Some clearly innocent laws have unintended discriminatory effects on some minorities. Accurately discovering the intent with which legislators acted is extremely difficult.

Judged by its effects, the commission system in Mobile clearly discriminates against blacks. They are not only kept out of office but are also deprived of any real participation in local government and , because their votes are in this sense meaningless, confront a government less than satisfactorily responsive to their needs. But judged by the intent of those who were around in 1911, the commission system does not discriminate against anyone. Blacks didn't vote in Mobile then. Besides, the system was adopted in Alabama, and else-where, as a reform of corrupt ward politics.

By opting for intent, or something close to it, a majority of the court has cut down dozens, perhaps hundreds, of legal challenges that would have been made against existing systems of government or multimember legislative districts. It has also avoided the logical terminal point of those challenges: that election district lines must be drawn to give proportional representation to minorities.

Those two factors offset whatever damage may have ben done to black hopes of using the legal system to open up all-white local governments. Not all problems of discrimination can (or should) be settled in the courts, and this is one left just as well in the political arena.