It seems trivial and I feel guilty when I begin thinking about what I should wear when I attend a funeral or memorial service yet when someone is close to you it is important to dress for them as a sign of respect. Whenever I wore a suit, my mother-in-law always complimented me and told me she liked suits on women. I'm going to wear a suit. She loved seeing her sons in suits and so of course, they will dress for their mom. And so it's not about you, when you decide on what to wear; it's all about the person you are honouring.
So is black the answer? Dark colours do seem to be the most acceptable particularly in European cultures but depending on your family ancestry and history, black may not be the choice. In my family, wearing black came with a set of rules depending on the relationship to you of the person who died. When my grandmother died, I was about eight and my aunt chastised my mother for allowing me to wear a red winter coat. As far as I remember, it was probably the only winter coat I had. I don't remember what my mother actually wore, but I do remember the black kerchiefs and scarves covering all the heads of the women in the church. For one year after the funeral, my mother wore black generally and she also wore a black armband on her coat or jacket to demonstrate that she was in mourning for her mother. After one of my brothers died, it was a very long time before she began wearing colour and it was only because, her other children began supplementing her wardrobe on birthdays and Christmas. In Canada, with so many different cultures living together there seems to be no need to be as demonstrative in mourning. In fact among my friends and family, if this custom ever was a consideration, it now has essentially disappeared. I wore beige to my eldest brother's memorial service in 2012.
Short of sleazy, I don't think it matters much what the mourners wear although we need to also be respectful of our surroundings. Like I said before, what we wear shows respect to the one we are honouring.