The answer to your question is yes - Cascara can cause this.Melanosis coli
is most commonly caused by laxative abuse. Cascara sagrada used to be classified by FDA as an over-the-counter laxative until November 2002 when FDA removed it because of some safety and effectiveness concerns. Since 2002, Cascara is sold as a dietary supplement (dietary supplements do not have to be approved by FDA).
Hope this helps.