There are actually two different sounds represented by the spelling "th" in English.
First you have the breathier sound found in words like thin, bath, third, and thud.
Then you have the less breathy sound found in then, bathe, them, and thus.
In Welsh, the spelling "th" only represents the first sound. The spelling "dd" represents the second sound.
So in Welsh,
Bleddyn and Blethin would not be said the same, because in the first you use the sound found in "bathe", and in the second you use the sound found in "bath".