The undergarments were un-dyed to begin with, just the natural organic cotton color which is a slightly off white. (For those of you with MCS { chem sensitivities} I found that these undies were not overly chemical smelling, and that AA is doing a fairly good job with their organic line. The smell they did hold washed out really easy for me.)
I believe there are various ways to prepare the elderberry dye - but what I did this time was cook it down with a lil' bit of apple cider vinegar and then let the cotton clothing sit in the dye bath overnight.
I am pretty certain you can get a MUCH darker color from this if you have tons of elderberries (which I didn't have anymore). Also, i think for a blue/ purple color that is dark possibly not cooking them at all, but crushing them up into a liquid, adding the vinegar and then putting the garments in would be really awesome, and likely a very deep dye.