I'm using the latest stable version 0.9.28.3 on one computer and yesterdays snapshot 0.9.29 version on another (both running on Windows 7) and in both versions the edit box part of the dblookupcombobox shows the correct font color but the dropdown part shows black font color.
So it's not a matter of upgrading Lazarus.
What operating system does your correct showing combobox run?
Maybe it works on Linux but not on Windows?

Jan