If creature A attacks, and creature B (who does not have flying) blocks it. Then creature A's controler gives it flying, its still considered blocked right?
506.4a (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=506.4a): Once a creature has been declared as an attacking or blocking creature, spells or abilities that would have kept that creature from attacking or blocking don't remove the creature from combat.
That's the rule.