31 cards where the card types include “land” and the mana produced is BU

Choked Estuary (Murders at Karlov Manor Commander #254)
Clearwater Pathway // Murkwater Pathway (Zendikar Rising #260)
Clearwater Pathway // Murkwater Pathway (Zendikar Rising #260)
Contaminated Aquifer (Dominaria United #245)
Creeping Tar Pit (Commander Legends: Battle for Baldur's Gate #888)
Darkslick Shores (Outlaws of Thunder Junction Commander #281)
Darkwater Catacombs (Outlaws of Thunder Junction Commander #282)
Dimir Aqueduct (Outlaws of Thunder Junction Commander #288)
Dimir Guildgate (Ravnica: Clue Edition #234)
Dismal Backwater (March of the Machine #269)
Drowned Catacomb (Outlaws of Thunder Junction Commander #290)
Fetid Pools (Outlaws of Thunder Junction Commander #296)
Frost Marsh (Coldsnap #146)
Ice Tunnel (Kaldheim #262)
Jwar Isle Refuge (Zendikar Rising Commander #133)
Mistvault Bridge (Modern Horizons 2 #249)
Morphic Pool (Commander Legends: Battle for Baldur's Gate #357)
Restless Reef (The Lost Caverns of Ixalan #282)
River Delta (Ice Age #359)
River of Tears (Murders at Karlov Manor Commander #283)
Salt Marsh (Eighth Edition #325)
Secluded Glen (Lorwyn #271)
Secret Passage (Ravnica: Clue Edition #20)
Shipwreck Marsh (Innistrad: Midnight Hunt #267)
Soured Springs (Outlaws of Thunder Junction #264)
Submerged Boneyard (Zendikar Rising Commander #141)
Sunken Hollow (Outlaws of Thunder Junction Commander #325)
Temple of Deceit (Outlaws of Thunder Junction Commander #328)
Undercity Sewers (Murders at Karlov Manor #270)
Underground Sea (Vintage Masters #323)
Waterfront District (Streets of New Capenna #259)
Watery Grave (Ravnica Remastered #291)

The message was clear to Sorin—time to search for something else.