Blank's solids are not over dyed. So they can regulate the exact color in their printing runs.
Many company's "solids" are the fabric that was used during first runs, printed to check for motif detail alignment, correct coloring of designs, etc. during the printing process, but aren't used for final production. Some gets sent back to the designer for approval at various stages.
When enough test runs have accumulated, it is over-dyed with black and sold.
If you do discharging of black (and some other dark) fabrics, you never know ahead what you'll get showing when the over-dye is removed. Art quilters love this.