The "huge black cloud" Sinbad refers to is most likely a Roc. In the story, the Roc is described as a massive bird, so large that it can block out the sun, creating the appearance of a dark cloud.

Here's why this is the likely explanation:

• Size and Appearance: The Roc is legendary for its immense size. The tales often describe its wingspan as being capable of obscuring the sky.
• Sinbad's Voyages: In "The Book of One Thousand and One Nights", Sinbad encounters a Roc in his second voyage. He sees what he initially perceives as a large white dome, which turns out to be a Roc's egg. When the Roc appears, its size would naturally give the impression of a massive, dark cloud.

Therefore, based on the context of Sinbad's voyages and the common descriptions of the Roc, the "huge black cloud" is almost certainly a reference to this giant bird.