I was thinking about suggesting maybe a fragment of neutron star since that would allow you to get the same mass in a much smaller area.
Unfortunately it seems the smallest size a neutron star could be and remain a neutron star is 0.1 stellar masses. That's 33 thousand times the mass of the earth so doesn't really work. (see https://physics.stackexchange.com/questions/143166/what-is-the-theoretical-lower-mass-limit-for-a-gravitationally-stable-neutron-st)
Average meteorites though have a density of 3g/cm. Make the meteorite out of something like gold or uranium and you can get the density up to 20g/cm. You should also make the meteorite spherical to get as much mass as possible for the cross section. Unfortunately square-cubed is not working for us here as a factor of 3 increase in density does not give us a factor of 3 decrease in cross section.
However it doesn't need to be a sphere. If you can control the meteorite's orientation and rotation finely enough you could shape it like a very long rod with one end pointed towards the earth. Make that end black and inclined at an angle so it doesn't reflect anything back towards earth.
You can now make the meteorite as massive as you like by extending the length of the rod without making it any more visible at all.
An important point to make though is that there is no need to camouflage it at all. We do not watch all the sky all the time, and not even all the meteors that we know are out there have been found yet. You don't need to explain a meteor not being spotted unless you want that explanation to be significant. It's actually more likely we would not spot one than that we would.