There are a couple of things to consider. The first is wearing the blue combat patch behind the wings as proscribed by the local regulations. This seemed to be something that was unique to the 8th AAF, but even within the 8th, not all groups seemed to follow these rules--you frequently see guys on combat duty NOT wearing the patch. It also seems that some groups in the 9th may have used these patches as proscribed.
Confusing this is the fact that many British-made bullion and embroidered wings were simply made on a blue backing. This was just something that was done at the level of the local manufacturers. As I understand it, the British used automated machines, where they would run a bolt of appropriate fabric (in most cases, a blue cloth melton type material) and embroider the wings directly on that (similar to what they did when they produced machine-made RAF wings). At some point, a tailor would trim the underlying blue material down and attach the wings to the uniform. This probably had nothing to do with the combat patch in many situations and is just an artifact of how the wings were being made.
For example, in the US, most of the embroidered insignia were made on gaberdine material. Thus, those wings had gaberdine backings. In Italy and the CBI, it seemed that they frequently used a black backing, so you see wings with black backing.
I think that is why you see many 8th and 9th AAF uniforms with nicely trimmed down wings with blue backing that were worn by guys who weren't necessarily in combat. Of course they had blue backing on their wings, the wings were made on blue cloth. I suspect that in many cases this differs from those guys you see wearing a rectangular blue patch behind the wings, as described in the regulations.