Yes, you might be right. It certainly could be either Black Sister. Delana might be acting under Halima/Arangar's orders. I guess Moria could be under orders from another Forsaken to provide information about Egwene but we really have limted screen time with her character. So IMO, she seems less likely to be the culprit.

I thought it was Mesaana because she appeared later and carried through on her threat to cut Sheriam's fingers off if she did not suceed in stealing all the dream ter'angreal. Mesaana also was the most active Forsaken in manipulating the "break" in the WT and was quoted at one of the Forsaken gatherings as having Egwene under control.