I suspect that this is due to the varying emissivities affecting your IR thermometer readings. I HIGHLY doubt that you are generating enough heat to make a foot-long aluminum bar vary by 27 degrees (15C). Blocks of Aluminum are 250 W/m*k thermal conductivity. If you've got 1/100th that, you'd need 70W. Read up on how the IR thermometer is affected by surfaces. You need special calibration (And surface selection!) to compare painted metal to shiny metal.
Yes, it is a common problem in pyrometry. I actually sell a high end "emissivity corrected pyrometer" sensor, and the effect is very large. (on the measurement, much less on the actual temperature).
Less obvious, is that the emissivity needs to be measured at the same wavelength as the pyrometer measurement or the results will still be substantially off, and it gets even more picky from there. It is even possible to have two paints that both look the same color to your eyes give completely different results from this effect.
These units are BMW sort of prices though, so definitely not for the consumer market.
If you are interested in making a black body radiator from copper, the most important thing is not the color, but the consistency of the surface. Consider rough sanding it, and then force oxidize it to a consistent look. (similar to anodizing aluminum) There are some artist techniques for copper that give that ugly green look - that is what you want.