Do you have another socket & bar holding the bolt on the topside? On the 2550 deck the hole in the blade is round – no star or other serration – that locks the blade to the spindle bolt. The spindle bolt is just compressed by the big nut to keep it in place. You can block the blade in place and usually the bolt and nut will just spin around together.
An impact wrench may loosen it, but if not you will need to get a socket on the bolt head topside to keep it from turning around. And yes, the head is too big for the socket to fit through the holes in the outside spindle covers – they have to come off (I am thinking about putting bigger holes in mine to avoid doing this). No, the owners manual does not properly describe this, but there are a number of things described in the manual that are just wrong.
If you are holding the bolt head topside and it still won’t come off, you do have a problem. It may be time for the pipe on the breaker bar mentioned above and/or the flaming wrench! Good Luck!