"Black Muslims" are most often different from mainstream Islam Muslims.  For example, in the Wikipedia article about Muhammad Ali (the boxer):  "As a Muslim, Ali was initially affiliated with Elijah Muhammad's Nation of Islam (NOI) and advocated their black separatist ideology. He later disavowed the NOI, adhering initially to Sunni Islam and later to Sufism, and supporting racial integration, like his former mentor Malcolm X."  ----- It might be the "black separatist ideology" part that concerns the border guards?