I think I'm turning into a signage anorak . I've found the "London Overground signs standard" online. 66 pages long - I'm in corporate design heaven!

There's no evidence of black text on orange background. The station fascias are shown as white text on orange background, and the directional and information signs are blue text on white, with orange stripe along the top (a la New Cross Gate). There's a section on platform roundels which are the LO roundel with the station name in white text on the blue bar (like LU stations have). The examples show New Cross and Peckham Rye, so perhaps the intention is to eventually introduce roundels on the platforms and remove the long oblong signs we have at present.

Still doesn't explain the awful black text through. I'm now on a mission to find out!