28 cards where the color identity ≤ BGU and it’s legal in Pauper and the text includes “whenever ~ becomes blocked”

Alley Grifters (Mercadian Masques #115)
Battering Ram (Fifth Edition #353)
Cave Tiger (Urza's Saga #241)
Deeproot Warrior (Ixalan #186)
Deepwood Wolverine (Mercadian Masques #242)
Drelnoch (Coldsnap #32)
Elvish Berserker (Tenth Edition #260)
Gloom Sower (Core Set 2021 #100)
Groffskithur (Mirrodin #121)
Ichorclaw Myr (Scars of Mirrodin #166)
Jungle Wurm (Vintage Masters #217)
Leery Fogbeast (Onslaught #273)
Norwood Warrior (Portal Second Age #140)
Phyrexian Reaper (Invasion #117)
Phyrexian Slayer (Invasion #118)
Plague Wight (Ravnica Allegiance #82)
Port Inspector (Mercadian Masques #90)
Pygmy Troll (Exodus #118)
Rabid Elephant (Odyssey #263)
Rabid Wolverines (Exodus #119)
Sacred Prey (Mercadian Masques #268)
Slashing Tiger (Masters Edition III #133)
Slate Street Ruffian (Jumpstart #279)
Snorting Gahr (Mercadian Masques #272)
Tel-Jilad Wolf (Darksteel #88)
Thresher Beast (Prophecy #128)
Vedalken Ghoul (Alara Reborn #32)
Zombie Boa (Apocalypse #54)

This search is finished. Now the real work can begin.