Seven days - This marks the duration of the plague. The natural discoloration of the Nile water lasts generally much longer, about 20 days.

And seven days were fulfilled - So we learn that this plague continued at least a whole week.
The contention between Moses and Aaron and the magicians of Egypt has become famous throughout the world. Tradition in various countries has preserved not only the account, but also the names of the chief persons concerned in the opposition made by the Egyptians to these messengers of God. Though their names are not mentioned in the sacred text, yet tradition had preserved them in the Jewish records, from which St. Paul undoubtedly quotes 2-Timothy 3:8, where, speaking of the enemies of the Gospel, he compares them to Jannes and Jambres, who withstood Moses. That these names existed in the ancient Jewish records, their own writings show. In the Targum of Jonathan ben Uzziel on this place they are called יניס וימבריס Janis and Jambris; and in the Babylonian Talmud they are named Joanne and Mambre, and are represented as chiefs of the sorcerers of Egypt, and as having ridiculed Moses and Aaron for pretending to equal them in magical arts. And Rabbi Tanchum, in his Commentary, names them Jonos and Jombrus. If we allow the readings of the ancient editions of Pliny to be correct, he refers, in Hist. Nat., l. xxx., c. 2, to the same persons, the names being a little changed: Esther et alia magices factio, a Mose et Jamne et Jotape Judaeis pendens, sed multis millibus annorum post Zoroastrem; "There is also another faction of magicians which took its origin from the Jews, Moses, Jamnes, and Jotapes, many thousands of years after Zoroaster;" where he confounds Moses with the Egyptian magicians; for the heathens, having no just notion of the power of God, attributed all miracles to the influence of magic. Pliny also calls the Egyptian magicians Jews; but this is not the only mistake in his history; and as he adds, sed multis millibus annorum post Zoroastrem, he is supposed by some to refer to the Christians, and particularly the apostles, who wrought many miracles, and whom he considers to be a magical sect derived from Moses and the Jews, because they were Jews by nation, and quoted Moses and the prophets in proof of the truth of the doctrines of Christianity, and of the Divine mission of Christ.
Numenius, a Pythagorean philosopher, mentioned by Eusebius, names these magicians, Jamnes and Jambres, and mentions their opposition to Moses; and we have already seen that there was a tradition among the Asiatics that Pharaoh's daughter had Moses instructed by the wise men Jannes and Jambres; see Abul Faraje, edit. Pococ., p. 26. Here then is a very remarkable fact, the principal circumstances of which, and the chief actors in them, have been preserved by a sort of universal tradition. See Ainsworth.
When all the circumstances of the preceding case are considered, it seems strange that God should enter into any contest with such persons as the Egyptian magicians; but a little reflection will show the absolute necessity of this. Mr. Psalmanazar, who wrote the Account of the Jews in the first volume of the Universal History, gives the following judicious reasons for this: "If it be asked," says he, "why God did suffer the Egyptian magicians to borrow power from the devil to invalidate, if possible, those miracles which his servant wrought by his Divine power, the following reasons may be given for it:
1. It was necessary that these magicians should be suffered to exert the utmost of their power against Moses, in order to clear him from the imputation of magic or sorcery; for as the notion of such an extraordinary art was very rife, not only among the Egyptians, but all other nations, if they had not entered into this strenuous competition with him, and been at length overcome by him, both the Hebrews and the Egyptians would have been apter to have attributed all his miracles to his skill in magic, than to the Divine power.
"2. It was necessary, in order to confirm the faith of the wavering and desponding Israelites, by making them see the difference between Moses acting by the power of God, and the sorcerers by that of Satan.
"3. It was necessary, in order to preserve them afterwards from being seduced by any false miracles from the true worship of God."
To these a fourth reason may be added: God permitted this in mercy to the Egyptians, that they might see that the gods in whom they trusted were utterly incapable of saving them; that they could not undo or counteract one of the plagues sent on them by the power of Jehovah; the whole of their influence extending only to some superficial imitations of the genuine miracles wrought by Moses in the name of the true God. By these means it is natural to conclude that many of the Egyptians, and perhaps several of the servants of Pharaoh, were cured of their idolatry; though the king himself hardened his heart against the evidences which God brought before his eyes. Thus God is known by his judgments: for in every operation of his hand his design is to enlighten the minds of men, to bring them from false dependencies to trust in himself alone; that, being saved from error and sin, they may become wise, holy, and happy. When his judgments are abroad in the earth, the inhabitants learn righteousness. (See Clarke's note on Exodus 4:21).

And seven days were fulfilled,.... Or there were full seven days, a whole week:
after that the Lord had smitten the river, and turned it into blood; here the miracle is ascribed to him; Moses and Aaron, and the rod they used, were only instruments, nothing short of almighty power could do such a miracle; it seems this lasted seven days at least. It began, as Bishop Usher (o) computes it, on the eighteen day of the sixth month, or Adar, part of February and part of March, and ended the twenty fifth of the same. It is not said that Pharaoh requested to have it removed, though Philo (p) says he did; his stubborn heart not being humbled enough as yet to ask such a favour, and therefore perhaps it was taken off without asking for it, to make way for another.
(o) Annal. Vet. Test. p. 20. (p) Ut supra. (De Vita. Mosis, l. 1. p. 617.)

Seven days were fulfilled - Before this plague was removed.