You see a magic wall.
Location
Paradox Tower, when using a Magic Wall Rune
Notes
It also exists in a yellow color:
.
On Open PvP and Retro Open PvP worlds, its color varies depending wether the caster and the viewer are on an active PvP situation or not. On Hardcore PvP and Retro Hardcore PvP worlds it's always blue whereas on Optional PvP worlds it will always appear be yellow when cast by players.
It is similar to the Rush Wood, although it blocks all kinds of projectiles and spells unlike the Rush wood. They can be destroyed by walking over them on Optional PvP worlds.
History
Before Updates/10.55 it looked like this:
The yellow version was implemented with the 10.00 Update.