{Urborg, Tomb of Yawgmoth}
{Blood Moon}
All lands are still mountains?
Yes.
4/1/2008: If Urborg, Tomb of Yawgmoth and either Blood Moon or Magus of the Moon are on the battlefield, all nonbasic lands will be Mountains, not Swamps. Urborg, Tomb of Yawgmoth will be a Mountain as well. It will have "{T}: Add {R} to your mana pool" and no other abilities.
http://magiccards.info/v12/en/13.html
All the nonbasics would be mountains, causing the tomb to lose it's abillity. The lands would no longer be swamps.
Wow, we posted 3 seconds apart.
Similar scenario came up at a legacy tournament (SCGLA)
One judged rules urborg was still present...
Oh man that guy was pissed who played {Magus of the Moon}
They had to call in a level 2/3 as they discussed the play like Refs in nfl
The reason that this is true (besides the fact that there is an actual ruling on this exact situation) is that when multiple continuous effects are applied in the same layer they are applied in time stamp order. This order may be interrupted by dependencies.
If we try to apply Blood Moon before or after Urborg, Blood Moon will affect the same number of lands. However, if we apply Urborg after Blood Moon, then Urborg loses its ability and affects no lands. That makes Urborg dependant on Blood Moon, so Blood Moon's effect has to be applied before Urborg no matter which has the older time stamp.
^ previous paragraph copy and pasted directly from the wizards website.