Here's why:
* The "i" in "blink" is followed by a consonant ("n"). Short vowels are typically followed by a consonant.
* The sound of "blink" is a quick, clipped sound. This is characteristic of short vowel sounds.
Compare "blink" to the word "bike." The "i" in "bike" is long because it is followed by a silent "e." The "i" makes a longer, drawn-out sound.