I found this table on a Japanese website this morning. It is a spectroscopic analysis of the mysterious Japanese substance at different sites all over Japan now, and coming to a pair of lungs near you soon.
It contains radioisotopes such as plutonium-241, cesium-137, uranium-235, gallium-57, cobalt-57, cobalt-60, manganese-54, and technetium-99.
This web page is an interactive spectrographic analysis tool which enables the user to analyze the black substance him or herself, using a mouse. Outstanding web programming here.