They would/should have been largely invisible; the edges of the patch were teased out, by the removal of a few of the threads, giving an edge (sort-of) reminiscent of a ragged bird's feather.
The patch would then have been doped on, usually with clear dope, followed by red, followed by the camouflage colour, with the edges of the dope being brushed out, to feather that in, as well. There might have been a slight hint, but that's all; larger patches would have been cut back to where the fabric was stitched, making them just as invisible.