Yeah, a lot of free blacks resided in the region and a number of Arabs also participated in the revolts it seems
Indeed, but you should check this out....
M. A. Shaban's
Islamic History: A.D. 750-1055 (A.H. 132-448) in the section "Regional Economic Conflicts":
"
All the talk about slaves rising against the wretched conditions of work in the salt marshes of Basra is a figment of the imagination and has no support in the sources. [...] The vast majority of the rebels were Arabs of the Persian Gulf supported by free East Africans who had made their homes in the region [...] If more proof is needed that it was not a slave revolt, it is to be found in the fact that it had a highly organized army and navy which vigorously resisted the whole weight of the central government for almost fifteen years.Moreover, it must have had huge resources that allowed it to build no less than six impregnable towns in which there were arsenals for the manufacture of weapons and battleships. These towns also had in their mammoth markets prodigious wealth which was more than the salt marshes could conceivably produce.
Even all the booty from Basra and the whole region could not account for such enormous wealth.Significantly the revolt had the backing of a certain group of merchants who persevered with their support until the very end. Tabari makes it very clear that the strength of the rebels was dependent on the support of these merchants."
"With remarkable efficiency and expedition the rebels swiftly established their control over most of the Persian Gulf coast, and extended it inland to secure their food supplies. Special vehemence was reserved for the port of Basra, which they practically destroyed.
Their choice of sites for their own new towns and their meticulous knowledge of the intricate waterways of the region in addition to their great skill in naval warfare were all utilized to strangle the Basran economy and drive all the in-coming trade through their own channels."