The life expectancy of blacks in the United States has lengthened since 1900, but a Texas researcher says the death rate for black infants is nearly double that for whites.

Dr. Kyriakos S. Markides of the University of Texas Medical Branch in Galveston said there were 23.6 deaths per 1,000 live births for blacks in 1977, as against to 12.3 per 1,000 for whites.

''This continued racial gap in infant mortality implies much progress can be made to further reduce the racial gap in life expectancy at birth,'' he said in a report in Public Health Reports, the journal of the Public Health Service.

The gap between the overall life expectancies of whites and nonwhites in the United States, the report said, has narrowed considerably from 14.6 years, which it was at the start of the 20th century, but whites still have a 4.8- year advantage in longevity. Longer Expectancy for Women