Just because your pill tested black doesnt mean there was no stimulant in them... a lot of the time pills will contain meth/amphetamine most commonly along with MDMA - its just the black will cover over the amphetamine.... I was even surprised myself when I had tested a pill I felt to be pretty pure have a little bit of meth in it! PURE MDMA isnt very stimulating, often meth is added to give it a bit of a stimulating boost not much but enough.
Anyway, I personally love a bit of meth with mdma - best combo for a long night out of fun...best for the brain well put two and two together. Deffintly potentiates the MDMA I feel, need less MDMA to be satisifed, I have tons of energy and never get floored.... I do notice that you heat up a fair bit more so keep on top of that, this is another supposed reason why meth+mdma is more toxic as it has been proven heat plays a role in oxidative stress plus the fact you have a mass amount of dopamine and serotonin running through your brain its neurotoxic end of story.