I think it is the reason they obliterated the Deguchi painting as well.
2. Remove his omoto underpinning
It should also be pointed out that Ueshiba was proud of his service in the Russo-Japanese war, and the overt purpose of the Black Dragon Society was to work toward the removal of the Russians from East Asia.
I would suggest that the original inclusion of the picture of a black dragon in a formal portrait taken at the Ueshiba Juku in the Oomoto compound at Ayabe (whether that piece of sumi-e was brushed by Deguchi or not) was originally intended as a visual suggestion that Ueshiba's art (and Deguchi's Oomoto project) was, at a minimum, in sympathy with the goals of the Black Dragon Society, and at a maximum, carried the Black Dragon Society's seal of approval; conversely, its removal from subsequent reprints of the picture was probably carried out in an appropriately cautious attempt to eliminate an obvious visual reference to prior political sentiments and associations on the part of both Ueshiba and Deguchi that clouded the post-war story line regarding both Aikido and Oomoto as "ways of peace."