In this week's Parsha, Hashem commands Moshe on the the first 7 of the 10 Plagues.

The first of the 10 Plagues is the "Plague of Blood."

The very obvious question is: why is this the first plague?

What's so special\unique about it - that it's the first one?

But the question intensifies, since in last week's Parsha, Hashem already commanded Moshe to turn water into blood, but that was as a "Sign" (אות) for Am Yisrael themselves, to make them believe that Hashem sent Moshe.

So what exactly is this "act with blood" - is it a plague?

Is it a sign for Am Yisrael? Are both actions - from last week, and this week - the same action of different ones?

What's the "Plague of Blood" about?