You would look for a yellowish color during the reaction with marquis, before it is overpowered by the dark blue MDMA result. I would also not grind the sample to dust but rather breaking it up in tiny pieces, leaving potential little clumps of bk-MDMA together, increasing the likelihood of you noticing the yellow color. Marquis normally has no yellow whatsoever with pure MDMA. The simons would be quite useless I think for this particular mixture as both bk-MDMA and MDMA give a blue reaction. With bk-MDMA it just comes on slower.
This is a difficult mixture to differentiate. Testkits are never 100% foolproof and I think with mixtures they really fall short if you don't monitor the reaction very very carefully and even then it's still not close to 100% foolproof. Far better than nothing of course