I agree that you should ask the neighbor what, if anything, was used. Its possible that there is still some herbicide residue under the blocks that is gradually leeching out.
If they did use an herbicide and you think it is trapped under the blocks, deep watering the driveway multiple times might drive it out. Of course you'll have to consider any local watering regulations before watering a driveway.
You might also consider other toxins that are spilled on the driveway. Fuel or car cleaning products might be causing the ongoing problems.
