Is it appropriate to wear a black dress to a wedding?

My ex-bf has asked me to going with him to his best friend's wedding! The only dress that I have is a black dress: Knee length, v-neck, sleeveless, with white design around waist area. It is truly a beautiful dress. He has seen the dress and he is telling me it is okay to wear it!! Should I? I have high heels and awesome accessories to wear with it!!