The BEM documentation suggests using two underscores to class elements (sub-modules in SMACSS terminology):
We use hyphen to separate words in long names (for example, block-name) and two underscores to separate the name of the block form the name of the element (block-name__element-name).
They do say "but you can use any other separators for it," but most implementations of it I've seen has followed the double-underscore method, since double-hyphens are used for modifier (variant) names:
A double underscore is for an element, and a double dash is a modifier.
And for instance see Nicolas Gallagher's post, under "Structured class names."
I love seeing the BEM methodology in Zen dev, but it seems like it's using double-hyphens instead of double-underscores for elements/sub-modules (e.g. "block--title" instead of "block__title").
If this is intentional that's fine, in which case this is just a documentation issue. But it wasn't clear to me.