Queen Victoria. The "Penny Black" is considered to be the first ever postage stamp. It was introduced by the British for entirely internal use in 1840. It was called the "Penny Black" because it was black and it cost one penny. The only design was Queen Victoria's head. Other denominations (and colours) were introduced once the idea caught on internationally.
Snehamol
My guess is Empress Zewditu of Ethiopia who appeared on a 1918 stamp of Ethiopia. You didn't ask from what country, so it's hard to say. If you are talking about the United States, the first Black woman to appear on a postage stamp was Harriet Tubman on Feb 1, 1978.
The first African-American woman honored on her own stamp was Harriet Tubman in 1978. Some of the letter-sorters shown on a 1973 stamp appear to black women.
US Scott #306 Martha Washington was the first American woman to appear on a US Postage stamp on that date. However, Queen Isabella of Spain was pictured in the Columbus series.
Martha Washington, wife of President George Washington.
Martha Washington.
MIRABAI
The very first postage stamp in the world pictured a woman, namely Queen Victoria in 1840. Except for some allegorical figures and namely Indian women, Queen Isabella of Spain was the first woman to be seen on US stamps.
Harriet Tubman
Harriet Tubman was the first to have her own stamp. Some black female postal workers appeared on an earlier stamp.
The first American woman to be featured on a US stamp is Martha Washington. The stamp was released in America in 1902.
St. Alphonsamma is the first woman from Kerala to be pictured on the Indian postal stamp.