It could be GPU or PSU. 700w psu is bare minimum for the 590. If its a lower end brand name psu it could be even less efficient for the system. Best way to test is swap out the components and test the system.
GPU death doesn't happen too much on the 590, but it's possible just like any other GPU. If unlocked for high overclocks or even worse Overvolting then your risks are much higher.
Yeah but the thing is I have been using the 590 for the last 5 months with my current 700W PSU fine before it died with these black screen crashes.
I also forgot to mention that I also tested my ram with memtest for a few hours with no errors.