25.2Why all three chords are dominant 7ths

This is already unusual.

In ordinary C major:

I would be:

Cmaj7

IV:

Fmaj7

V:

G7

But blues uses:

C7, F7, G7

So even the tonic carries a ♭7.

C7:

C E G B♭

Yet C still feels like home.

This is one reason blues doesn't behave like ordinary functional major harmony.