JOEL - Here is the scripture in question:

"And the God of our fathers, who were led out of Egypt, out of bondage, and also were preserved in the wilderness by him, yea, the God of Abraham, and of Isaac, and the God of Jacob, yieldeth himself, according to the words of the angel, as a man, into the hands of wicked men, to be lifted up, according to the words of Zenock, and to be crucified, according to the words of Neum, and to be buried in a sepulchre, according to the words of Zenos, which he spake concerning the three days of darkness, which should be a sign given of his death unto those who should inhabit the isles of the sea, more especially given unto those who are of the house of Israel." (1 Ne. 19:10)

The prophet Zenos is prophesying about what those living on the "isles of the sea" would experience at Christ's death, which would include the Nephites. He further specifies that the sign of three days of darkness would be recognized "more especially" by those of the "House of Israel" (who inhabit the isles of the sea), not the land of Israel. The Nephites were considered a part of the "House of Israel".
Jacob, refering to the allegory of Zenos, and conscious that his people branched from the house of Israel, proclaimed:

"And how merciful is our God unto us, for he remembereth the house of Israel, both roots and branches; and he stretches forth his hands unto them all the day long;" (Jacob 6:1-4)

The Nephites were one of those branches of the "house of Israel" as explained in a Biblical prophecy about Joseph from whom Lehi decended:

"Joseph [is] a fruitful bough, [even] a fruitful bough by a well; [whose] branches run over the wall:" (Genesis 49:22, See also 1 Nephi 5:14)

As you probably know I have already explained the reason for the different lengths of time of darkness(3 days vs 3 hours), at the following page:
http://www.mormonhaven.com/scriptur.htm#19