Testicular Trauma: This is the most likely diagnosis due to the direct correlation between physical injury to the testicles and the appearance of bruising. The mechanism of injury, such as a direct blow to the testicles, supports this diagnosis.

Epididymitis: Inflammation of the epididymis, which can be caused by infection, can lead to swelling and bruising of the testicles. Symptoms such as pain, swelling, and discharge would support this diagnosis.
Orchitis: Inflammation of one or both testicles, usually caused by bacterial or viral infections, can result in pain, swelling, and potentially bruising of the testicles.
Varicocele: A varicocele is an enlargement of the veins within the scrotum, similar to varicose veins. While it might not directly cause bruising, it can lead to discomfort and changes in the scrotal appearance that might be mistaken for bruising.

Testicular Torsion: Although less common, testicular torsion is a medical emergency that requires immediate attention. It can present with sudden, severe pain and swelling, and potentially bruising if the torsion leads to ischemia and necrosis of the testicular tissue.
Testicular Cancer: While rare, testicular cancer can present with a painless testicular mass. In some cases, if the cancer leads to bleeding or necrosis, it might cause bruising. Given the potential severity of missing this diagnosis, it's crucial to consider it.

Henoch-Schönlein Purpura (HSP): A form of vasculitis that affects blood vessels and can cause bruising, including in the genital area. It's more common in children but can occur in adults.
Scrotal Hematoma: A collection of blood outside of blood vessels in the scrotum, which can be caused by trauma or spontaneously. It might present as bruising and swelling of the scrotum.
Spermatic Cord Torsion: Similar to testicular torsion but involves the spermatic cord. It's less common and can present with acute pain and potentially bruising due to compromised blood flow.