As a matter of longstanding practice, and supported by W3C's draft HTML5 specs, along with my own sense of 'the rightness of things', I place citations/attributions outside of blockquotes. Blockquotes, by my view, contain only HTML markup and quoted material. So, if I'm quoting (at length) material from another source, by convention I place the quoted material in the blockquote and the attribution/citation outside the blockquote. In Markdown, this looks like
> Lengthy quote.
(source of lengthy quote)
The relevant verbiage from W3C is
The blockquote element represents a section that is quoted from another source.
Attribution for the quotation, if any, must be placed outside the blockquote element.
Here a blockquote element is used ... to clearly relate a quote to its attribution (which is not part of the quote and therefore doesn't belong inside the blockquote itself)....
The above is an example of my style choice. In my view, placing the citation/attribution inside the blockquote (other than in a cite attribute or similar markup, as discussed below) not only looks wrong, it violates the spirit and the letter of HTML5.
The HTML5 recommendation from 2014 differs slightly from the Editor's Draft, but the recommendation is clearly geared toward (a) future browser implementations, rather than present-day implementations, and (b) a version of Markdown that implements the recommendation and is appropriately translated for rendering by common browsers. That is, the 2014 recommendation, as opposed to the draft quoted above, assumes the implementation of appropriate rendering for elements not currently implemented either by Markdown or by common browsers:
The blockquote element represents content that is quoted from another source, optionally with a citation which must be within a footer or cite element, and optionally with in-line changes such as annotations and abbreviations.
Content inside a blockquote other than citations and in-line changes must be quoted from another source, whose address, if it has one, may be cited in the cite attribute.
Attribution for the quotation, may be be placed inside the blockquote element, but must be within a cite element for in-text attributions or within a footer element.
(W3C HTML5: A vocabulary and associated APIs for HTML and XHTML, W3C Recommendation 28 October 2014, "4.4.4 The blockquote element". Bold emphasis mine.)
So, the question is, have I overlooked an ELU convention for blockquotes and blockquote attributions (that is, a convention that both the blockquote and its attribution should be put inside Markdown that will be translated into one blockquote element) supported by sufficient reasoning to overcome my objections (as suggested by the foregoing)?
A 'question' on Meta SE also broached this topic, so I added an 'answer' synthesizing this 'question' and the accepted 'answer'. See Allow sourcing in blockquote markdown.