Blues music is often considered to be "black music" because it has its roots in the African American communities of the Southern United States. The origins of the blues can be traced back to the late 19th and early 20th centuries, when African Americans were facing the challenges of poverty, racism, and social marginalization. The music that developed in this context was heavily influenced by African rhythms, harmonies, and melodies, as well as the European-American musical traditions that were present in the South. 

One of the most notable characteristics of the blues is its use of the "blue" notes, which are microtonal inflections of the pentatonic scale. This use of blue notes gives the music a unique, soulful quality that reflects the emotional experiences of the people who created and performed it. 

Blues music was originally performed by African American musicians, often in rural communities and at "juke joints" (small, informal music venues) and later in cities like Chicago and Memphis, the music was amplified by electric instruments and became urban blues. 

These early blues musicians, who often had to contend with poverty and discrimination, sang about their struggles, hopes, and experiences, and in doing so, they created a powerful and enduring musical genre that has had a major influence on all forms of popular music, including rock and roll, R&B, and hip-hop. 

So it is because of its origins, the musical traditions that influenced it, the themes it addresses and its creators' identity that Blues music is considered to be black music.