If you're purblind, you aren't able to see very well. This word is also used figuratively to describe someone who lacks insight or good judgment, like your purblind uncle who always gives foolish advice.
These days, it's quite old-fashioned to describe someone as purblind, although you may come across the word in old books. When it first appeared around 1300, it was two words, pur blind (possibly from pure) and it meant "entirely blind." Later, it was used to mean "a blind person," and then "partially blind" — like your elderly, purblind poodle, who has become almost entirely dependent on her sense of smell.