The crud that sticks to the inside of lead pots is oxidised metal and only a smelter can recover it back to metalic form. In other words it won't get into your alloy, except for possible small droplets of unoxided metal, which probably wouldn't effect your black powder bullets in the least unless you had heavy visable lead clinging to the pot surfaces.
So just scrape the empty pot out, fill it with what you intend to use and go.
If you can't live with that or feel certain some serious contaminant is in the pot, sand blasting will clean it to the pots base metal. don't sand blast a Lee pot or you'll probably have leaks.
