If a physicist uses E=mc^2 in some research, is he stealing? Isn't everything in human culture based on something that came before? I am stealing these words by using them?
Good artists copy, great artists steal.
The point is that if you copy something, it's still someone else's work. You've just made a lifeless derivative. If you steal it, however, that implies more than making a copy - it implies making it your own. Things stolen no longer belong to the original owner.
An example: House of the rising sun is a traditional song, passing round loads of musicians - but the Animals "stole" it in such a way no-one can hear it anymore and not think of their work.
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.