The correct option is B I have never seen a watermelon with blue flesh.
An adjective phrase must be placed immediately after the noun it modifies.
From the options, B has the correct placement, "with blue flesh" is the adjective phrase that modifies the noun "watermelon". Hence option B is correct.
Option A is incorrect because the word added to the given sentence, "before" is neither an adjective phrase nor an adjective; it's an adverb.
Option C is incorrect because the adjective phrase is placed before the noun it modifies.
Option D is incorrect because the answer lies in B.