It's 8 inches high...are these rare? ...
Not rare, Blair, but not common either. Davidson only seem to have made matt black in smallish quantities, and at the two extremes of their manufacture of the 8" 279. The early matt black from the late '20s is typified by the 291 Tutankhamun square bowl. The late matt black from the late '50s is typified by the 518 oval footed bowl, made from a modified late pre-war Grecian mould.
I don't know to which of these two periods of matt black your vase belongs, but I think it much more likely to be the earlier. This view would be strengthened if the 6" or 10" 279 were known in matt black, as these two sizes had a shorter production period than the 8".