It is not easy to find positive factors in the unfortunate contretemps that the Senate Judiciary Committee finally made of the Clarence Thomas confirmation hearings. There is one "plus," however, that is well worth noting: Witness after witness, including Prof. Anita Hill herself, presented black Americans who were dignified, sincere, well-educated, literate, and well-spoken people. They were lawyers, professors, judges, and teachers - people of real importance.All too many Americans still harbor stereotypes of black Americans. This display of real talent and accomplishment should go far to dispel these long-outmoded misperceptions. Albert F. Gallistel, Wayzata, Minn. Letters are welcome. Only a selection can be published, subject to condensation, and none acknowledged. Please address them to "Readers Write," One Norway St., Boston, MA 02115.