The block mixin creates both a class and a SCSS placeholder so the pattern can be extended (for rationale read this article) . It also adds the name to a global list and gives a warning if there is a name conflict. The passed name will be used to prefix to all the element and modifer mixins.
There should be only one block definition per file. Big patterns, ie. comments should be split into separate files (and blocks) with a single index file importing them and setting up any variables, placeholders etc.
A common problem is the context of patterns. If some content is in a pod, it might have slightly different styling. Sometimes a modifier to the pattern will be enough, ie. ".-inside-pod" but sometimes the context itself need extra properties to accomadate the pattern.