Having said that, the OP is not talking about that "quote"-- frequently attributed to Picasso, but probably from T.S. Eliot (Thanks Nancy Prager: http://nancyprager.wordpress.com/2007/05/08/good-poets-borro...) Maybe Picasso stole that too. I think people know the quote from Steve Jobs' interview (mis)attribution? to Picasso.
Anyway, I think your point is essentially then of copying something without attribution or credit to the point of destroying the original authorship, and I would have to say that that from of "stealing" is also wrong.
Your blues music is a good example of stealing, literally the use of copyrighted songs written by impoverished black artists by artists like Led Zeppelin without paying royalties.
So a clarified version of my point is: Using an idea and making your own expression or extension of it is not stealing, it is the basis of all knowledge and culture. Copying someone's expression of an idea, whether legal or not, without attribution when you know who you are copying and you are able to give attribution, is wrong.