i agree that u cannot tell, esp if it wasn't a pure mix but i do have this theory.
safrole (precursor to MDMA, and no, hehe, not admitting to testing safrole myself, got info off hive) which has no amphetamine structure turn the reagent PURPLE then to black. this is coz it contains the MD part of the MDXX drugs that differentiates them from speed etc.
we know amphetamine and methylamphetamine turn the reagent orange/brown. what about ethylamphetamine, the other half of MDEA. from what i understand this is useless as a drug goes (which could go a fair way in explaining y MDEA has little amphetamine like effects). Perhaps ethylamphetamine doesn't change orange with the tester, or to a less extent.
THUS:
the MD part of the molecule gives a purple reaction.
the amphetamine/methamphetamine part of the molecule gives orange. those two colours together would give a darker dirtier black colour. (e.g saw an old XO go more browny/dirty colour then str8 to black, no purple at all, and we all know about them)
the ethylamphetamine in MDEA gives no reaction and hence y MDEA is just purple coz its only reacting to the MD part.
its just a stab in the dark, don't have a clue how the reagent reacts to ethyl-.. but it could explain y they did say initially MDA/MDMA blue/greyish -> black, and MDEA purple -> black