Boshi refers to the temper line that extends to the Kissaki (the tip of the Katana). It is a huge job to bring out the Hamon on the tip, as it requires an extremely long polish. It takes about the same time to polish the tip as it does to polish the rest of the blade. Impressive isn't it?
A beautiful Boshi will be polished until it is obscured and barely visible to be an important, but not distracting detail. In general the Boshi forms a sort of comma continuing the Hamon line and going back to the tip.
Only differentially hardened swords have a visible boshi.