Ohh ok i understand, but maybe some notation in docs to this method so other won't miss the point of what they can achieve with it. Thanks @md_5 like always 
The sky light on a block will be always be 0.
You want the sky light above a block - ie: it only makes sense to get the skylight of transparent or AIR blocks.
I need to check if any block face in on sky light. So i was wondering if methods above check that on all faces because thinks it should.
But what is a difference of this: player.getLocation().getBlock().getLightFromSky() with this player.getTargetBlock((Set<Material>) null, 6).getLocation().getBlock().getLightFromSky(). When we use location class so nothing should different.
You probably want: Block.getRelative( BlockFace.UP).getLightFromSky()
player.sendMessage("LO5:" + player.getLocation().getBlock().getLightFromSky()); - works
player.sendMessage("LO:" + player.getTargetBlock((Set<Material>) null, 6).getLightFromSky()); - not
player.sendMessage("LO:" + player.getTargetBlock((Set<Material>) null, 6).getLocation().getBlock().getLightFromSky()); - not
Edit: If i will use method form block model not accessed from event it's works. So just when i use for ex. BlockPlaceEvent and getBlock and then light level it's won't work.