I've seen that sort of munition before. To me it looks like an old 50lb inert training bomb. They were used mainly during the 50's and 60's as practice bombs for light attack aircraft. They were originally painted blue as all inert training weapons are.
That said, it could still be packed with explosive as they were hollow bodied, and detonated with a mortar fuse with a minimum of modification. Or the simple method of an electric cap shoved into the nose and fired electrically.