there are a blood moon, an Urborg, Tomb of Yawgmoth , an island.

can this island produce {B} ? why ?

Hi,

Thanks for your question. Both Blood Moon and Urborg want to apply in Layer 4 (type-changing effects). But Urborg's effect is dependent on Blood Moon's effect, since applying Blood Moon will change what Urborg will apply to (by removing its ability):

613.7a An effect is said to “depend on” another if (a) it’s applied in the same layer (and, if applicable, sublayer) as the other effect (see rules 613.1 and 613.3); (b) applying the other would change the text or the existence of the first effect, what it applies to, or what it does to any of the things it applies to; and © neither effect is from a characteristic-defining ability or both effects are from characteristic-defining abilities. Otherwise, the effect is considered to be independent of the other effect.

So no matter what order the two cards enter the battlefield, we always apply Blood Moon's effect before Urborg's effect. We apply Blood Moon's effect first, making all nonbasic lands (including Urborg) into Mountain. This removes Urborg's ability and now it only has the ability to tap for one red mana. Since Urborg no longer has its ability, it won't apply, and other lands will not get the swamp subtype.

The net result here is that all nonbasic lands are Mountains and no land will gain the Swamp subtype.

Nathan Long
Magic Rules NetRep