So the first option is just to add some more "unflavored" tea to tone it down. If you have a flavored black tea, maybe Lapsang Souchong, and the smoky flavor is a bit overpowering, just mix it with some regular black tea like a Orange Pekoe. In my college days we used to do that a lot. Going to college in Scotland we made our tea in a teapot, using loose leaf tea not with individual tea bags for each person, so it was important to get a mix that everyone liked! I guess some of our friends didn't like the Lapsang Souchong so much, so we made it less potent by mixing it with black tea. Lapsang Souchong is a bit of an acquired taste, kind of like drinking tea while being at a bonfire and inhaling the smoke! So most of the time we didn't make a pure Lapsang Souchong pot of tea. Of course, that might also have been a way to save money, Lapsang Souchong being more expensive than regular black tea and we were poor students on a limited budget! But, I like to think we were motivated by a desire to share something we really liked (the smoky flavor of Lapsang Souchong tea) with all our friends.
Another option is to mix two flavored teas together. Here my ideal is to mix Lapsang Souchong with Earl Grey tea. While I love them both, I have to admit that most Earl Grey tea is a little bit like drinking perfume! That bergamot oil that gives it the special flavor is actually used for perfume too so that's not surprising. So, rather than just tone it down by adding more black tea, a more exciting option (particularly if you're partial to the smokiness of Lapsang Souchong) is to add a portion of the Lapsang Souchong to the brew. Now that's a delicious pot of tea!