Why does God bless Christians despite their not understanding that Islam is the correct religion?

No, the worldly blessings with which Allah graces the disbelievers are not a blessing from Him, the Exalted. Rather, they are a gradual enticement (istidraj) for them, as Allah Almighty says: "We will progressively lead them to destruction from where they do not perceive. And I will grant them respite; indeed, My plan is firm." and His saying, the Almighty: "And whoever disbelieves - I will grant him a little enjoyment, then I will compel him to the punishment of the Fire, and wretched is the destination." This is also mentioned in the prophetic hadiths, such as his saying, peace and blessings be upon him: "Indeed, Allah grants respite to the oppressor, but once He seizes him, He does not let him go," and his saying: "The world is a prison for the believer and a paradise for the disbeliever."