The spelling of /b/ is always either <b> or <bb>. The /Ɵ/ sound is always spelled <th>. I have excluded several pairs retrieved by the computer in which the sounds occurred at different places in the word, such as broth/throb.
The contrast is between a voiced stop and a voiceless fricative, fairly well separated in the mouth. It is unlikely to be a listening problem, although many speakers will have difficulty articulating the /Ɵ/ sound if it does not occur in their own language.
The only pair involving a taboo word is buggery/thuggery .
Interesting pairs include
The mean density value is 1.2%. The list makes 43 semantic distinctions, a loading of 60%.
arbour Arthur bane thane bank thank banked thanked banking thanking banks thanks barb bath batch thatch batches thatches bawd thawed beam theme beams themes beery theory beef thief beeves thieves Bertie thirty bicker thicker bin thin bird third birds thirds boar thaw boars thaws board thawed bole thole boles tholes borax thorax bore thaw bored thawed bores thaws borough thorough bought thought Bourne Thorne brash thrash brawl thrall brawls thralls bread thread breaded threaded breads threads breeze threes brew threw brew through brill thrill brush thrush brushes thrushes bud thud budded thudded budding thudding buds thuds bug thug bugs thugs buggery thuggery bum thumb bummed thumbed bumming thumbing bums thumbs bump thump bumped thumped bumping thumping bumps thumps burst thirst bursting thirsting bursts thirsts buy thigh buys thighs cab Cath deb death dub doth gob Goth lobe loath mob moth mobs moths rob wrath
John Higgins, Shaftesbury, January 2010.