Why do we read u as (“u”) in “blueberry” and as (“a”) in muffin?
That´s easy to remember.
In closed syllables, “u” is read as ” ʌ “. a
And what is a closed syllable? Well, when the vowel is followed by one consonant at the end of the word, and by two consonants in the middle of the word, then the syllable is CLOSED.
For example, muffin, cup, nuggets, bug, multi.
But if it is followed by a vowel, or by a consonant and a vowel again – like in music, pure, cute – the syllable is OPEN, and u is read as in the alphabet.
Of course, there is an exception to the rule. In the case of the “ur” combination, followed by a consonant, it is pronounced as ɜː
Nurse, burn, curl, hurt…