What is cause of blob when layer area reduces by 50%?
Try lowering your extruder flow rate and maybe temperature as well.
I had this problem once and I thought it was just a bit of gunk that had been hanging onto the outside of the extruder. That is usually not the case and instead it is probably build-up from over-pressure inside the nozzle.
Also note that 250 °C will start degrading your PTFE tube unless you have an all-metal extruder. Lowering temperature can also reduce the stringing visible in the picture, however to fully, get rid of stringing, you may have to adjust your retraction and acceleration.