Interesting; I've long intended to use cans for similar purposes, but I'm thinking white to reflect rather than black to absorb.
My interest is mostly in growing things against the southish facing side of a container, and I think the reflected light will be more useful than absorbed heat due to the (relatively, compared to say a stone wall, or terrace with soil behind it) low mass of the container; no matter how hot you get that few thousand pounds of steel, it's going to be damned cold long before morning.
The possibilities get even more interesting if you have a pond or reservoir to play with... Solar heating devices benefiting from both the reflection of a pond and absorbtion/reflection of a container ought to be extremely effective. Plus a lighter container could also reflect light onto the pond, further increasing the temperature of that thermal mass...
(Also, how is absorbtion not a word? Bullshit.)
If one is using multiple containers, maybe the southernmost could be white, and the others dark, so that you still have some dark rooftop space and end-wall space, but less extreme heat-cycles inside the containers... Or, if you are planning to roof over the container/s, a bit of end or side wall could be left with less overhang to maximize sun exposure for this.
Containers are very strong, but that strength is not evenly distributed. They are meant to take huge amount of weight at certain points along the walls. The roof is meant to withstand a heavy snow load. If very heavy containers of water are to be placed on the roof, they must be placed in the right locations.
Very important to keep this in mind! However, I'd expect support of a modest reservoir at the right spot to be pretty straightforward; a hundred gallons of highly insulated water storage on top of the container could be heated via thermosiphon by collectors along the side/end of the can; valving to close off flow at night would allow preservation of the heat collected.