Unity candles are usually pillar candles and are sometimes decorative - my aunt and uncle’s unity candle is black and white and carved - and is accompanied by two tapers. The candles, depending on your beliefs, represent one of two things (or both): the individuals coming together as one in marriage, or the flame of the candles can represent the Holy Spirit or Jesus bringing His light to the marriage. Honestly, I’ve not attended a wedding where the latter is the case.
The ceremony itself takes place, generally speaking, right after the exchange of vows and rings. It can be part of secular wedding ceremonies, though it is used in Christian weddings as well. The bride and groom each take a taper candle that has been lit either by themselves or, more commonly, by a parent or elder, and then light the unity candle together.
But do you blow out the unity candle? Well, sure. But not until the wedding is over and the newly wed couple are long gone down the aisle, this is normally done by the officiant or another wedding attendant. Another route to take would be to get very symbolic and blow out only the tapers showing that the bride and groom are no longer individuals but a new entity: a married couple.
Due to it being a relatively new custom and therefore not part of the church’s apostolic tradition, this ceremony is not always allowed in church weddings . So to be safe, always ask your officiant/pastor if they will administer the unity candle ceremony.
According to Wikipedia, unity candles and their ceremonies did not appear until the latter half of the twentieth century, and were seemingly popularized in the 1980s by being included in an episode of General Hospital.
The theme of the unity candle has been used in derivative ceremonies as well; one of my friends got married and used her favorite coffee and her husband’s favorite coffee as the tapers and poured them into a glass vase to represent their coming together in marriage.
Even though this is a newer tradition, given the popularity of it and its off-shoots, I would like to think it’s here to stay. It seems indicative of a new attitude toward marriage, an emphasis on partnership with your best friend and becoming one as opposed to the old order of your spouse being your partner but not necessarily your friend.