Why does Allah blind some people and then judge them for being blind?

This must be important, as it is stated so early in the Quran at Surah 2:7. Why does Allah blind people to the Truth prior to judging them on their acceptance of the same? > Allah has set a seal upon their hearts and upon their hearing, and > over their vision is a veil. And for them is a great punishment. Also 2:17, > Their example is that of one who kindled a fire, but when it > illuminated what was around him, Allah took away their light and left > them in darkness [so] they could not see. On my human understanding, this seems unjust on its face. Obviously, God is the source of all justice, so whatsoever He decrees is Just by definition. Nevertheless, why would He set a certain standard of justice in our hearts and minds and then deploy a different standard of justice vis-a-vis the above verses? Considering one of his famous names is the Merciful, why would he give us a challenge to solve with the tool of rationality which He also gave us and then so clearly traduce it, as above?