In Hardmode, the evil biomes are capable of converting nearby Mud Blocks to Dirt Blocks, and Chlorophyte Ore is capable of doing the reverse. While the evil biomes can also convert Mud Blocks covered by Jungle grass into a Dirt Block with the respective evil grass type, Mud Blocks with Mushroom grass can never be converted.
Thrown Water and the Clentaminator similarly convert Mud Blocks covered with Jungle/Mushroom grass. Bare Mud Blocks will not be converted by these items, unless there is a grass block of the target biome (e.g. a Crimson grass block when using Blood Water/Red Solution) directly adjacent to it.
↑Information taken from the Desktop22.214.171.124 source code, method Convert() in Terraria.WorldGen.cs. There may be inaccuracies, as the current Desktop version is 126.96.36.199.