BEM block, naming & nesting

There are a number of variants on how to write BEM classes, so be aware that are multiple competing standards. It started as a set of loose guidelines. Since posting this answer, Yandex has significantly overhauled their official standard (it’s quite an improvement). The version of BEM I use is based heavily from an article by Nicolas Gallagher.

At this point I use “Atomic OOBEMITLESS”, which is really just a way of saying that classes are modular and namespaced, selectors have low specificity, and states may be toggled with classes which allows CSS to be scaled, on top of a CSS preprocessor to make the code more concise and expressive.

All that said, I will be using the following BEM standard:

• hyphenated class names as blocks:
  foo-bar
• block name followed by __ followed by hyphenated class names for elements:
  foo-bar__fizz-buzz
• block or element names followed by -- followed by hyphenated class names for modifiers:
  foo-bar--baz, foo-bar--baz__fizz-buzz, foo-bar__fizz-buzz--qux

BEM short form: block-name__element-name--modifier-name

You have three different blocks in your examples:

1. sidebar, which has a sidebar__nav element
2. content, which has a content__nav element
3. nav, which has nav__item and nav__link elements

The sidebar and content blocks appear to be variations on the same block, and could be represented as .region.region--sidebar and .region.region--content.

The nav block is implicit on the ul element, and you should make it explicit:

<ul class="nav"><!-- could be content__nav or sidebar__nav as well if you choose -->
    <li class="nav__item"><a href="#" class="nav__link">LINK</a></li>
    <li class="nav__item"><a href="#" class="nav__link">LINK</a></li>
</ul>

Once you’ve specified that the ul element is a nav block, it makes sense to have the nav block be modified:

<ul class="nav nav--content">
    <li class="nav__item nav--content__item"><a href="#" class="nav__link nav--content__link">LINK</a></li>
    <li class="nav__item nav--content__item"><a href="#" class="nav__link nav--content__link">LINK</a></li>
</ul>

Once you’ve set up the CSS classes, styling all nav__item elements is simply:

.nav__item {
    ...styles here...
}

and overriding those styles for the content nav__item elements is:

.nav--content__item {
    ...styles here...
}