I think "Block Capitals" is a more common term, and it's been around for as long as there have been "forms", so they would likely originate around 3000 BC in south west Mesopotamia.

Perhaps there has been a conflation, and often what is meant by "Block Letters" is, in fact, "Block Capitals." I just wish that when the State of Massachusetts mandates them, I knew what they were talking about.

Why bother at all about Blockheads?
Why should you care what they do?
’Coz after all is said and done
You’re all Blockheads too!

Best damn response I've had all year. cheers.

Block Letters is also a style of lettering (pen & brush) that just means a plain sans serif letter (unburdened by the grot/human/geo classifications of type).

We get requests for block fonts in our designs at work all the time. I usually end up giving them something like Rockwell extra bold, a slab serif, and yes, always in all capital letters. The customers seem happy.

I thought the term "block" derived from the slabs themselves actually, i.e., block fonts = slab serif fonts, though I could be quite wrong.

Perhaps, but a cursory browsing through Google books seems to show that after the 1840s the term "wood-block letters" becomes shortened to "block letters," and is distinct from "gothic letters;" the block type usually having angles restricted to the octagonal, and the gothic being fully realized sans-serif. There doesn't seem to be any consistent restriction of block letters to capitals until shortly after the turn of the century, and primarily in technical drawing alphabets. And by then the distinction between the block and the gothic is becoming blurred.

Ryan - it seems that the term "block" originally derived directly from the wood they were carved from, though by the end of the 19th century the name is ascribed to the method of composing the letter forms from grids made of "blocks."

At the risk of beating horses already passed, a little more digging has found that as of the 1830s the term is applied to letter forms that have been produced in some form of relief, whether in iron, marble or cut from a wooden block. The term "block" seems to have, therefore, derived from the letters having an actual three-dimensional form.

makes sense.

The Athenaeum, Issues 375-426, 1835

All images below from "A Textbook On Lettering" published out of Scranton, PA, 1902.

Those have been digitized by Letterhead fonts

And then there are these block letters ...