At the build limit of 255, you cannot normally place solid blocks, but can still place torches, redstone, water, saplings, lava, and the like. Even though you can't directly place blocks, there are still a couple ways to produce blocks at that level, such as growing a tree up to that level, or placing lava and water next to each other to produce obsidian or cobblestone. While an opaque block at 254 will obstruct a beacon's beam, one placed at 255 does not block the beam. Quite trivial, but there you go.