This link shows the ferry routes from US trans-Africa to the Far East. Date was 30 June 1942. I have a more detailed one of the Accra-Cairo one if you want it.
Regards
Nick

http://www.ibiblio.org/hyperwar/AAF/I/maps/AAF-I-11.jpg

Air Corps Spec. 24114 (Oct. 22 1940 ) approved Insignia Blue ....
" E.4a. The marking U.S.ARMY on the undersurface of the wing shall be applied with blue, Shade 47 of Bulletin 41 ."  This was ANA605 Insignia Blue in Bulletin 157 ( 28 Sept. 1943).
The marking was ordered removed from May 29 1942 and from training aircraft Oct. 19 1942. Obviously all lettering was not deleted immediately but aircraft in production were no longer required to have it applied.
It would be difficult if not impossible to tell from photos ( even colour ones) whether some planes had black instead, no doubt that colour was used for convenience in some situations.
Nick