In a bit of a predicament (Black Water tank full and frozen)

You can rent a kerosene heater from most national hardware stores. They are sometimes called bullet heaters and are used extensively by contractors for outdoor situations. Needs to be plugged in to operate but takes very litlle electricity. The blast from the heater will flood the area of the frozen tank and should reduce or eliminate the issue. Good Luck.
Buy the way you can own a homeowner version of the heater for around $150.00. Home depot, True value, Lowes to name a few.