Halakhah su Isaia 50:3

7. The Rambam in the laws of Tzizith Chapter II Halacha II gives in the sign to recognize the Hillazon and this is his holy words. Its a fish whose color resembles that of Techelet and whose blood is black as ink. I have found no source in the Talmud in support of Rambam words that its blood is black as ink. And it appears from this that after Rambam wrote his commentary on the Mishnah he toiled and labored until he found the Hillazon according to the signs mentioned in the words of our Sages of Blessed Memory and fulfilled the Precept Mitzvah of Techelet. Then he proceeded to enumerate other signs that could be found in it such as its black ink like blood. He though that perhaps it would be forgotten this would make it easier for one who desired to search for it and be able to find it without giving up. For if Rambam did not make it known to us (regarding its black blood) we would search for it relying solely on the signs in the Talmud. And even if we would find it we would lose hope saying that since its blood is black this is certainly not the Hillazon needed to dye the Techelet, Therefore he made it known to us that in truth its blood is black and only by adding of the substances is it possible to dye it the color of Techelet.

R. Joshua ben Levi said, "If one sees the sky in all its purity, he says, 'Blessed is He who has wrought the work of creation.'" When does he do so? Abaye said, "When there has been rain all the night and in the morning the north wind comes and clears the heavens." And they differ from Rafram bar Papa quoting R. Hisda. For Rafram bar Papa said in the name of R. Hisda, "Since the day when the Temple was destroyed there has never been a perfectly clear sky, since it says, 'I clothe the heavens with blackness and I make a sackcloth their covering'" (Isaiah 50:3).