I have had three blocks crack in the same way. One of them is still in use. So yes, you can definitely keep going, just check it periodically and if the crack starts to run all the way through or changes direction and heads for the top of the block, then it's time to replace it. There are definitely some strong forces at work in that little part. The bearings alone are a tight fit and put pressure on the block, especially that thin wall where the crack usually starts. The filament exerts force outward against the idler. Eventually your idler will likely start to crack from that force so print a few spares. And the motion of extrusion and retraction will create force and the gearing amplifies the torque. In short, there is a lot of motion going on.
Interestingly, a Wades block printed for me by William Frick has out lasted all others and so far hasn't developed any cracks. It's not as pretty as nophead's block and I think that might be part of the reason. The perimeters and infill are not as tightly bonded which might allow the block to flex just a little bit...at least that's my current theory. It may also be because I have never used that machine for PLA which, from my experience, is harder on machines. I'm not using PLA much any more so we'll see if the blocks tend to last longer on both machines now.
