Let me ask you a question how many km on the car? Did you service this car regularly?

But for them to say that because you have blue smoke, not white right? That you need a new engine sounds like B.S. to me, but you should be able to find a used replacement motor for way less than that, The labor is still gonna cost you the same at Nissan. But as your car is old and out of warranty you can get anyone willing to do the work for you.