From the description and looks I would think it is highly possible that it is in fact a Black Arkansas.
Density Standards
Soft Arkansas**** **********2.20-2.30*** Specific Gravity
Hard Arkansas*********** * 2.30-2.45****Specific Gravity
True Hard Arkansas***** *2.50 +** * ****Specific Gravity
Colored Translucent**** * 2.50 +**** ****Specific Gravity
Black Arkansas************ 2.50 +**** *** Specific Gravity