First Plague: Blood  מכת דם
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 וַיֹּאמֶר ה' אֶל-מֹשֶׁה, אֱמֹר אֶל-אַהֲרֹן קַח מַטְּךָ וּנְטֵה-יָדְךָ עַל-מֵימֵי מִצְרַיִם עַל-נַהֲרֹתָם עַל-יְאֹרֵיהֶם וְעַל-אַגְמֵיהֶם וְעַל כָּל-מִקְוֵה מֵימֵיהֶם--וְיִהְיוּ-דָם; וְהָיָה דָם בְּכָל-אֶרֶץ מִצְרַיִם, וּבָעֵצִים וּבָאֲבָנִים.  וַיַּעֲשׂוּ-כֵן מֹשֶׁה וְאַהֲרֹן כַּאֲשֶׁר צִוָּה ה', וַיָּרֶם בַּמַּטֶּה וַיַּךְ אֶת-הַמַּיִם אֲשֶׁר בַּיְאֹר, לְעֵינֵי פַרְעֹה, וּלְעֵינֵי עֲבָדָיו; וַיֵּהָפְכוּ כָּל-הַמַּיִם אֲשֶׁר-בַּיְאֹר, לְדָם.  וְהַדָּגָה אֲשֶׁר-בַּיְאֹר מֵתָה, וַיִּבְאַשׁ הַיְאֹר, וְלֹא-יָכְלוּ מִצְרַיִם, לִשְׁתּוֹת מַיִם מִן-הַיְאֹר; וַיְהִי הַדָּם, בְּכָל-אֶרֶץ מִצְרָיִם 
       שמות ז: י"ט-כ"א --   

And the LORD spoke unto Moses: 'Go in unto Pharaoh, and say unto him: Thus saith the LORD: Let My people go, that they may serve Me: And if thou refuse to let them go, behold, I will smite all thy borders with frogs: And the river shall swarm with frogs, which shall go up and come into thy house, and into thy bed-chamber, and upon thy bed, and into the house of thy servants, and upon thy people, and into thine ovens, and into thy kneading-troughs: And the frogs shall come up both upon thee, and upon thy people, and upon all thy servants.
-- Exodus 7: 19-21

The Plague:
The first plague that God brought upon the Egyptians due to the Pharaoh’s refusal to let the Israelites go is blood. God instructed Moses to tell his brother Aaron to hit the Nile and turn its waters to blood. Since the plague affected the Nile River which was instrumental in hiding Moses he could not be the one to strike it, since he would be hurting the very thing that saved him. Upon the striking of the Nile all waters used by the Egyptians no matter what the source was turned to blood. Even water that was collected prior to the plague turned to blood. The waters of the river remained blood for seven days, leaving the Egyptians with undrinkable water, dead fish and a terrible stench pervading the land.

Why were the waters of the Nile turned to blood?
Nile River was Egypt’s source of wealth and economy as well as vital for its existence. Pharaoh and the Egyptians therefore worshipped the Nile as a deity. Some Talmudic commentators note that the plague of blood was God’s smite against the Egyptian deity.

Water of the Nile turning to blood was also “Measure for Measure,” as retribution for their cruel behavior, as some observe that this plague was punishment of the Egyptians who used the waters of the Nile to spill Jewish children’s blood, as they followed the Pharaoh's decree to drown the newborn Jewish baby boys in the river.

The king’s magicians were able to duplicate the feat and so Pharaoh who relented at first and allowed Israelites leave, hardened his heart and reversed his order.