I wonder if you can scrape enough gravel/dirt/sand/rocks from the surrounding area and cover over the exposed concrete pad?
That way it would be identical, actually IS, to the surrounding area, and it would be no different than driving a post in and securing your temperature/humidity sensor to that.
I know you don't want anything dark or heat absorbing but in this case, rock is rock.
I think that a few inches of stuff over the concrete would also effectively isolate it from the sun's energy, heating up in the day and then cooling more slowly at night.
I know a neighbor here in the midwest greenery galore and not much of your surface, who puts big rocks for landscaping purposes in his garden, only to discover that they cool slowing after the sun goes down providing a wonderful warmer niche for his hostas. You'd being doing the opposite by keeping the huge mass at ambient temperature.