I tend to procrastinate a lot and I think it's mostly because my attention span is really short. So I work on something for a while, then take a 'short' break and then that break turns out to be longer because whatever I did in that break was too interesting 
I've only ever had a real art block once and the difference showed quite plainly: When I'm procrastinating, then I'll be able to continue writing by just reminding myself 'You should be doing that chapter' and I can sit down and do the rest. When I had art block I couldn't do that. Even if I did sit down, I couldn't get anything done or even if I started something new, I always stopped after a few chapters and never was happy with anything. That condition lasted for more than a year. I was unable to even finish one story in that entire year and was just a step short of calling it quits completely.
In hindsight, even though I was severly depressed during that time, I probably needed that forced 'break'. That art block was both my body and mind telling me that I couldn't continue how I did before and that I needed to change something or I'd be paying a price. In the end, while I thought I was wasting that year away, I learned lots of different new things that I probably would have never considered if I had continued writing throughout that year and it also gave me the courage to abandon everything I did before and start over afterward.
So overall, I think I want to say that even having an art block (or procrastinating for that matter) isn't inherently bad even though we might not be able to see that while we're in the slump. It's just something we'll have to suffer through and make the best out of like any other situation and in the end, we might have unexpected gains.