Mix the milk and condensed milk in a large bowl until well combined. Add a third of the blackberries and crush on the bottom of the bowl slightly to break up and release some juice to color.
Divide the remaining blackberries between 8 lolly molds, then fill until almost complete with the liquid. Freeze for one hour or until firm enough to allow a lolly stick to stand up in the mixture, then add the lolly stick and continue to freeze until solid.
When ready to serve, dip the molds into a pot of warm water to gently release the lollies. Serve immediately.