but if someone is reading the full article on your blog they have no need to view the original. yes it's difficult to police but it is still a breach of copywrite and the author of the content is well within their rights to ask you to remove it or take legal action.
But aside from any of that it's not good for steemit to be full of unoriginal material and seeing as we all have a vested interest in steemit doing well, it's best to avoid copying full works and putting your own voice to the topic with quotes and links anyway.