Is my understanding correct, that all objects emit a perfect black body spectrum only dependent on its temperature,
This is the theoretical black body curve. This is modified by the type of material the body consists of, because the underlying level is quantum mechanical and photons have to be absorbed and radiated from specific energy levels, depending on the type of material.
but we cant measure this perfect spectrum because all objects will reflect some of the radiation distorting the reading for the wavelength of radiation reflected.
It is not just reflection, it is also quantum mechanical transitions for bodies consisting of different atomic and molecular structures.
The energy radiated from a black body is given by the Stefan-Boltzman law, for grey bodies, i.e. not ideal black bodies, an emission coefficient is used dependent on the materials, in order to fit observed emissions.
The best experimental fit to the black body formula is the cosmic microwave backround radiation:
Graph of cosmic microwave background spectrum measured by the FIRAS instrument on the COBE, the most precisely measured black body spectrum in nature. The error bars are too small to be seen even in an enlarged image, and it is impossible to distinguish the observed data from the theoretical curve.
The solar spectrum as an example is approximately a black body curve,(the yellow histogram) because the sun is an active, continuously generating power by fusion , with a lot of plasma surrounding it, which also distorts from the black body curve.
