When in the nether/a nether fortress, blazes start firing at a player inside hallways/in another room/in a tunnel/even completely surrounded by blocks... all before the player moves into an open area and SHOULD be seen by the blaze. Fire balls don't penetrate the walls/nether fences, but this makes "sneaking up" on a blaze much more difficult since it already knows where you are and is throwing fire balls in regular intervals.