From your description it appears that there is blue discolouration of only lower limbs whenever there is stasis of blood flow either under effect of gravity or when the baby is held in such a position that the blood flow back to heart is impeded. Even though the report for echo is normal, get him examined if blood flow through to lower limbs is normal. Otherwise what I gather from the details you have given, baby looks fine and this should settle over a period of time.