I had this same problem after applying the widescreen patches after installing this mod due to a missing widescreen-patched blaster icon. My fix was to manually put a proper icon file named iw_blstrpstl_099.tpc for Carth's blaster into the swotor\Override folder, since this restoration mod's .uti file uses the 99 variant instead of the usual 1-6. There may be a better way to do this, but I installed the "Pretty Good! Icons for KOTOR" mod, linked below, for the hi-res icons after applying the widescreen patching (followed this https://www.wsgf.org/article/star-wars-knights-old-republic guide):
^ The Pretty Good mod provided a widescreen icon file for the standard 6 blaster variants in the swotor\Override folder - these are named "iw_blstrpstl_00[1-6].tpc" e.g. "iw_blstrpstl_002.tpc". The original Carth's blaster per the original g_w_blstrpstl010.uti file uses blaster variant 2 (file ending in 002) so I just made a copy of Pretty Good's widescreen patched iw_blstrpstl_002.tpc file and renamed the copy iw_blstrpstl_099.tpc. This fixed the white square for me. The equipment screen now showed the original Carth Blaster icon while retaining this restoration mod's unique in-game models.
My Override folder still includes all the 4 files from this restoration mod + my newly added tpc file:
w_blstrpstl_099.mdl
w_blstrpstl_099.mdx
w_blstrpstl_099.tpc
g_w_blstrpstl010.uti
(new) iw_blstrpstl_099.tpc
For convenience, I've attached the iw_blstrpstl_099.tpc file here. Again, credits to ajdrenter from the Pretty Good! Icons mod since I just renamed their blaster icon file.
I suspect if you aren't using any widescreen patching that you could do the same thing just copy the original iw_blstrpstl_002.tpc file into your swotor\Override folder and rename it to iw_blstrpstl_099.tpc. I figure this would work with any other blaster icon mods -- just take your preferred icon file and rename it to iw_blstrpstl_099.tpc.
iw_blstrpstl_099.tpc