In general, I'd say that's fine. It's probably not a bad idea to disclose the original source (whether blockquoted or not), but this is less due to concerns of integrity and more just to prevent the post from being mistakenly flagged as plagiarism if the original is discovered.
As I see it, for the purposes of this site the biggest problems with plagiarism and copy-paste articles in general are,
- Votes should reflect actual expertise: Typically, the only actual expertise shown in copy-paste is "I can find a webpage on the internet" which, while useful, usually doesn't warrant the types of votes such answers attract. This is exacerbated with plagiarism, where the expertise of the poster is confused with the expertise of the original author and the votes are inadvertently cast for the wrong person (the author, rather than the poster) entirely.
- Posters should be able to clarify their posts: Answers that aren't clear to the person asking the question are pretty much the definition of "not useful", and seeking clarification is one of the valid uses for comments. If the questioner (or a future user with the same question) doesn't understand how the post actually answers his question, the poster should be able to clarify it further (possibly by editing the post itself; see point 4 below). However, I find that when a post is copy-paste it's rare that the poster actually understands the points in the post well enough to actually explain them to others beyond the copy-paste itself.
- Posts should be open to criticism: Constructive criticism is an important aspect of the peer-driven system that makes Stack Exchange credible (as well as being one of the other valid uses for comments), but copy-paste articles are typically immune to it. We can't criticise the author, or his methodology, or his facts, or how he presents his information, or pretty much anything, because he's not the one who posted it. The poster has no real authority (or, in most cases, expertise) to actually respond to or handle any form of criticsm beyond "Copy-paste answers are bad, mmmkay?".
- Posts should be editable: Similar to point three above, the ability of the community to collaborate and improve posts over time is one of the major reasons Stack Exchange is so successful as an experts platform. But beyond trimming out the excess, there is very little one can do to edit a copy-paste without pretty much destroying it.
- Answers should actually answer the question: This one is pretty self-explanatory. Often, copy-paste "answers" were written for a completely different purpose than what they're posted as (i.e. answers); even the ones that are copy-pasted from another Q&A site are rarely answering exactly the question asked here.
When dealing with copy-paste that the poster had actually written himself, points one through four are not a problem: The votes would reflect the poster's actual expertise, he would be well-able to clarify his own points, he'd be able to act on criticism and his post would be editable.
The only potential issue is with point five above; the answer should actually answer the question. But that's really no different from any other answer, and unlike a third-party copy-paste it can actually be worked to answer the question if it's not already a perfect fit (or downvoted/flagged/deleted if it's not worth the effort).
In other words, it can pretty much be treated exactly like any other answer, and judged on its own merits.
