Question

Why the following if statement succeeds ?

if $(ps aux | grep -q "bla bla") ; then echo "found" ; fi

Answer 1

Because the grep process itself is being returned by ps. You can “trick” grep to not match itself by surrounding one of the search characters in a character class [ ] which doesn’t change the functionality:
Just do:

if ps aux | grep -q "[b]la bla" ; then echo "found" ; fi

Also, the use of process substitution $() is unnecessary. The if will work on the success of the last command in the pipe chain, which is what you want.

Note: The reason the character class trick works is because the ps output still has the character class brackets but when grep is processing the search string, it uses the brackets as syntax rather than a fixed string to match.