Why doesn't blood sound like \ˈblüd\? [duplicate]
An attempt to a historical explanation could be found in the Great Vowel Shift. Before the Great Vowel Shift that started around the 14th century "oo" was pronounced [oː] and it then evolved into [uː], see chart. Evolution continued and some words evolved in different ways and are now pronounced differently. But as to why they have evolved differently I expect we can only make guesses. About "flood" and "blood", nowadays pronounced with a short /ʌ/ we know that at one point in time they both used to be spelt respectively floud and bloud, could that be a reason why their pronunciations have evolved in a similar manner?
You might find those posts on linguistics.stackexchange interesting:
https://linguistics.stackexchange.com/questions/2085/how-do-linguists-determine-at-which-point-the-great-vowel-shift-was-complete
https://linguistics.stackexchange.com/a/4181/637
or this page on the internet in addition to the one given in my first paragraph:
http://marymoore2012.weebly.com/1/post/2013/04/english-as-a-global-language-great-vowel-shift.html
and this page from The History of English Spelling
Many vowel pairs have more than one sound
- oo has at least three: hoot, hood, flood.
- ou has at least three: pound, soup, double.
- ea has at least three: peal, bread, break. (In fact, the word read uses two of these: I read a book yesterday; I will read another book tomorrow.)
This gets even more varied when the vowel pair precedes an r or gh
- Consider poor, earn, pear, court, scour
- When ou is followed by gh, things get ridiculously varied: thought, though, through, tough, cough, bough
Let's not forget about when w functions as a vowel
- That's why words like bow and row rhyme with BOTH know and now. ("Tie a bow, then take a bow," for example).
Some of these pronunciations vary by region
- In the U.S., some areas of the country pronounce "route" as a homophone of "root" (which rhymes with "toot"), while others pronounce it as a homophone of "rout" which rhymes with "out").