Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
I haven't had the need to do it, but I'd test print a clear base or curable reducer. If that wasn't dark enough, I'd tint the base slightly with black or dark grey. (This assumes you're printing on light grey shirts, as in the photo.) The other step to let the shirt color show through is print a single pass on a high mesh. Hard duro squeegee can help, too.