That doesn't look like spray paint to me, the edge between the blue and black is too well defined and the wavy look has something of the hand done to it.
There were special dyes sold for shoes, to me it looks like it's something like that. If so I would try meths. Try a bit on the hidden edges. Other solvents to use are acetone (as has been suggested) or isopropyl alcohol.
I wonder if it was done by someone who just wanted to 'modernise' it? Black was a popular colour in the 80s.
Whatever the colour it's a very nice radio
