Modern Madness continues, if you hadn’t caught onto this week’s theme. Since the manabases in Modern can get greedy with their shocks and fetches and other color-providing wonder, Blood Moon is a common sideboard hate card to punish those greedy manabases. A whole board of colorfixing goodness, turned into nearly worthless Mountains for the low low price of 2R! But how does it interact with weird lands, like Dryad Arbor, the Forest whose bark IS its bite?
Well, to find out, we look at the layers! Blood Moon does its work in Layer 4, where it sets the subtype of “Mountain” to the exclusion of all others. That has some built-in rules baggage, though; setting something to a specific Basic Land subtype (like “mountain”) without specifying ‘in addition to’ means that it blanks out every ability on the card and replaces it with “T: Add R to your mana pool”. The card can still gain other abilities in layer 6 (for example, if the Dryad Arbor gained abilities from an Equipment or Enchantment or somesuch, those would stay, because they’re added in layer 6, but the ability to tap for G is removed in Layer 4 by having it set to Mountain). It’ll still be a Creature, but instead of Land Creature – Forest Dryad, it’s a Land Creature – MOUNTAIN Dryad, and the only ‘native’ ability it will have is tapping for Red mana.