Is there a standard way to add a blurb to an epub file, and to Amazon files, such that the information will show up on an ereader device?
Anything that you want to actually show up on an ebook reader will need to be in the content files of the epub, just like any of the chapters or other divisions of the text. It is possible to include a description in the metadata of the book; this should appear in the
<meta> section of the content.opf file and be formatted as described in the epub 3.0 specification. That metadata will not necessarily be visible to any reading systems, but it'll be in there.
Note that the
<dc:description> element uses the Dublin Core Metadata standard, which says
Description may include but is not limited to: an abstract, a table of contents, a graphical representation, or a free-text account of the resource.
So while there's some room for interpretation, it's not really the place for blurbs as such, which are generally endorsements rather than "account[s] of the resource."
Again: if you want it to be read by devices, just include it as another section of the manuscript.
What you call a "blurb" has actually a long tradition in publishing, is, or should be, a presentation, a comment written by the publisher. In France they use a very nice word: "prière d'insérer" referring to a text prepared by the publisher and aimed to booksellers and general readers. I believe such a text can find a right position immediately after the summary and before the beginning of the main content. Anyway don't forget to put something also into the "description" tag of metadata section of .opf file.