My block was, and still is in some spots, painted blue. Tho mine is not the original '51 216 as far as I can tell. When I started repainting parts of it as I have panels off I've been using Rust-Oleum New Ford Gray 500*F engine enamel.

Here's a picture of mine.
http://farm7.static.flickr.com/6230/6211277329_de7a038329_b.jpg