Calyth, depends on your size. I find that slipping my BK7 sheath and all into the small of my back makes it disappear, if I were in the concealing business, I'd definetely consider an appropriate sheath for this kind of carry.
IMHO there are 4 things that affects concealability, in order from greatest to least:
1. Dress style, if you wear loose or baggy, you can obviously conceal alot more, I tend towards jeans and big, long, flowing open-style shirts, which I find very good for concealing dang near anything I feel like concealing, including a BK7 (with a little creativity).
2. Personal dimensions, obviously, big people can carry bigger stuff, i'm 6'5", big-boned, and clock the scales at a respectable 320lbs, so I can carry a great deal of large things discreetly (such as a 3 D-Cell Maglite, 6" Barreled Large Frame Revolver, and oh by the way, a BK7)
3. Sheath/Holster, it's really important that your sheath or holster make use of the advantages within the countours of your body, sheaths that just "hang out" are rarely going to generate the best concealability, you need to really experiment and understand what works with your body, I find the small of my back often a convenient place to put very large objects I don't want to advertise having.
And finally 4. The actual item to be carried. Haha! Bet you thought this would be more important, actually no. Unless whatever you're carrying is excessively bulky or massive in proportion to your personal dimensions(which I don't characterize the BK7 to be for most anyone over about 5'5"), size matters little provided you make the best use of the first 3 priorities. If you do encounter problems, you can make small alterations to your carried item, such as removing the handles on the BK7 and going for a cord wrap situation.
Maybe I'm preaching to the choir, but I thought I'd share my insights in the hopes that they could be of some use.