When I think of "Black Tie Affair", I think of men in tuxedos and coat-tails and women in long gowns and diamonds...something like the Oscars.
If you Google "black tie affair", you find this definition:
"Black tie affairs are social events where all attendees are expected to wear formal attire. Both males and females attending a black tie affair make efforts to comply with the dress code and choose clothing that is considered appropriate formal dress for the time of day, the season, and the nature of the event. A black tie affair is different from a semi-formal event, where there is a wider range of clothing options considered to be appropriate semi-formal attire."
Notice that there is a very clear distinction between a black tie affair and a semi-formal event.
If you go on NAP, they have a whole section of dresses dedicated to black tie affairs and NONE of these dresses are knee length. Why? Because knee-length (or even tea-length) dresses are considered to be semi-formal attire and semi-formal attires are only reserved for semi-formal events and NOT black tie affairs.
**End of rant**