Arab seafarers had named the Indian Ocean as Bahr-el zanj. What does it mean?

I think this translates into English as Sea of Zanj, the Arabic name for the region of the Indian Ocean near the coast of Africa.

Although geographically insufficiently defined, the area of the Sea of Zanj included a vast maritime region roughly stretching as far as ancient navigators reached on their dhows. Subject to Monsoon wind variations, the Sea of Zanj was located to the south of the Erythraean Sea. It stretched off the coast of Southeast Africa as far south as the Mozambique Channel, including the Comoros and the waters off the eastern coast of Madagascar. On its eastern side the sea extended west of the Seychelles and to the northwest of the islands of the Mascarenes.

https://en.wikipedia.org/wiki/Sea_of_Zanj

Zanj was a term describing both the region and the people who lived there.

https://en.wikipedia.org/wiki/Zanj

The word, Zanj, to the arabs actually translates{roughly} to the "country of the blacks". It was a term used by medieval Arab/Muslim geographers and sailors alike to refer to certain parts of what is today known as East Africa{mainly the Swahili coast and adjacent lands}.

The word Zanj was also deployed when referring to the bantu inhabitants of this region.

So, to go straight to your question, Bahr-el Zanj would roughly translate to the "Sea of the land of the blacks"(mouthful) or "Sea of the blacks". That is if translations are to be strictly adhered to.

Note: Bahr-el Zanj is a former name for the western portion of the Indian Ocean adjacent to the shores of south east Africa. It wasn't a general designation for the Indian Ocean. For certain reasons, the Arab sailors were really scared of the western portion of this ocean hence the need to mark it out specifically.

The terms Bahr-el Zanj and Zanzibar both share similar origins.

The word Zanzibar came from Arabic zanjib?r [zand?iba?r]), which is in turn from Persian zangbâr [zæ?ba:r]), a compound of Zang [zæ? ], "black") + bâr [b?:r], "coast"),[14][15][16] cf. the Sea of Zanj. The name is one of several toponyms sharing similar etymologies, ultimately meaning "land of the blacks" or similar meanings, in reference to the dark skin of the inhabitants.

https://en.wikipedia.org/Zanzibar