Definitely go to the funeral...and like others have said, it really doesn't matter much what you wear. My mom died unexpectedly in Dec 2004 (exactly three weeks before Christmas, talk about timing). We were living in New Orleans at the time, but were originally from FL. We actually had two services, a cremation and small service in NOLA, then one for the family in FL. I cannot stress to you enough how important is was to us that each and every person showed up. My mom's cousin in Blioxi, who we hadn't spoken to in at least 10 years, came first, as he was the relative in closest physical proximity to us. Then my mom's sister, with whom she had had a very contentious relationship over the years came from FL to stay with us for a few days. My mom passed away while camping in a state park, and the campground hosts came to the NOLA funeral (and became extremely good friends with my Dad, they still talk on the phone once a week). At the FL funeral, we weren't sure what to expect, as we had been away for five years. But the church was packed!! My mom's old work friends, people who attended the church she was raised in....I can't remember everyone specifically, but it meant SO much to us to realize that all those people took time out to remember her. We still have all of the cards and letters that were sent as well. And honestly, I couldn't begin to remember, much less care, what a single person was wearing.
Go, speak to the parents, and also send a letter or card. Wear whatever you feel comfortable in, and know that the family will appreciate it more than you can imagine. I'm sorry for your loss.