I write because I want to, and also because I can. Would I write if I had no readers? Yes, but I'd certainly write something entirely different!
And I write because I have a readership who, if I want their loyalty and interest, equally deserve mine.
I have had spells when I haven't written for a while (I'm seriously ill, it's not always possible), but being blocked is never a problem. Nor, really, should it be for a blogger.
For a professional writer it's different - novelists can very easily write themselves into a corner and find themselves blocked, it kinda goes with the territory for every author at some point. And that's understandable. A blocked blogger, though - nope, I just don't get that.
For me there's a whole world out there, just beyond my keyboard, to rummage around in, and on any given day there will always be something that will spark my interest and creativity. I don't mind if I have to spend a lot of time reading online - if inspiration won't come to me, I'll go looking for it. I've been writing online for six years now, and I can honestly say that I've never been stuck for something to write about.
It helps, of course, not to give your blog a name that ties you to a specific subject - that'll seriously cramp anybody's style (especially if there are many thousands of bloggers writing about it). But, if you can't write about whatever your chosen subject is, assuming there is one, then write about something else - some of my most successful posts have been written just on a whim - you simply cannot predict what will take off on any given day. So try something new, it might surprise you.