Combine the avocados, blueberries, dates, milk and lime juice in a high speed blender. Puree the mixture until it is smooth. Divide the mixture among the ice pop molds until almost filled to the top. If using the nut butter, push the nut butter into the center of each ice pop. Freeze the pops until frozen solid, at least 8 hours. Remove the pops from the molds and enjoy!