You can simply consider the following grammar $$ S\to S_1 b S_1$$ where $S_1$ is the start variable of a grammar for the language of words $w$ with $|w|_a = |w|_b$. Correctness is self-explanatory.

So you have the following grammar: $$ S\to T b T$$

$$ T \to aTbT| bTaT | \epsilon$$

You can simply consider the following grammar: $$ S\to S_1 b S_1$$ where $S_1$ is the start variable of a grammar for the language of words $w$ with $|w|_a = |w|_b$. Correctness is self-explanatory.

So using the grammar you wrote, you get the following grammar: $$ S\to T b T$$

$$ T \to aTbT| bTaT | \epsilon$$