Great to see this idea being promoted without having to do it myself! :-))
However, I should point out that Higher Order Messaging and blocks (or ActionExpressions) are distinct mechanisms. Blocks are not an implementation of Higher Order Messaging.
In fact, HOM was specifically developed as an alternative to blocks/closures/higher-order-functions in a language where blocks aren't available, and the name was chosen to make clear that this is a higher-order mechanism based on messaging instead of (anonymous) functions.
A presentation held at MacHack a while back gives some more background info, though it is admittedly a bit terse: http://www.metaobject.com/papers/HOM-Presentation.pdf.
Furthermore, it does seem a bit odd that the original and definitive implementation of HOM, MPWFoundation, isn't mentioned.