This is an interesting question. I have copied below an abbreviated extract of the OED entry on 'sanguine' (missing out most of the quotations). You should find it interesting. The earliest meanings in the dictionary, which I have not included, relate to 'blood redness'. - see Susan's answer for an explanation as to how sanguine became one of Hippocrates' four humours.
The original meaning of 'sanguine' as you rightly deduce is to do with blood and redness and that was largely how it was once used. However because the blood red complexion became associated with hopefulness, 'sanguine' came to be the way of describing that particular humour.
I must add that I too have difficulty associating the two meanings. But the word has such a splendid history that it is worth cultivating.
sanguine, adj. and n.
Forms: ME sangueyn(e, sangweyn(e, ME–15 sanguyn(e, sangwyn(e, ME–17 sanguin, ME sangewyn, ...
Etymology: < French sanguin (feminine sanguine ), < Latin sanguineus
4 a. Of persons or their dispositions: Having the mental attributes characteristic of the sanguine complexion (see sense A. 3 above); chiefly, disposed to hopefulness or confidence of success.
4 b. Of persons and expectations, etc.: Hopeful or confident with reference to some particular issue.
1673 R. Allestree Ladies Calling (1684) Pref. 4 When the most sanguine of his Disciples had denied, yea forswore, and all had forsaken him.
1876 A. J. Evans Through Bosnia ix. 417 And yet how fascinating is Ragusa still! It far surpassed our most sanguine expectations.
†1. A cloth of blood-red colour, also a piece of this.
1612 H. Peacham Gentlemans Exercise xxiii. 86 With which water you may diaper and dammaske vpon all other blewes, and sanguines to make them shew more faire and beautifull.
†3. The sanguine ‘complexion’ or temperament.
1530 J. Palsgrave Lesclarcissement 265/1 Sanguyn a complexion, sanguin.
1718 G. Hickes & R. Nelson Mem. J. Kettlewell i. ii. 15 His temper was a Mixture of Sanguine and Choler.