Bleeeet Bleeeet Bl Bleet!
Retina, 31 bytes

T`lL`e
(?&lt;!e)e
B
Be
Bl
e(?!e)
t


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Explanation

T`lL`e


Turn all letters into es.

(?&lt;!e)e
B


Turn the first e in each run into B.

Be
Bl


Turn Be into Bl.

e(?!e)
t


Turn the last e in each run into t.