A blade is any sharpened edge made with the intention of cutting things (but we guess you already knew that).
'Blades' in the SCA
Live steel (any real edged weapon) is strictly forbidden on the field by the rules of the list. If you are going to be using a knife or similar tool in a way that could hurt anyone you should make sure that no one is nearby and warn anyone who is (this seems obvious but accidents do happen). Some people in the SCA like to say "clear!" when they unsheathe blades near others.
For heavy fighters in the SCA the "blades" on weapons used in combat are in fact strips of tape in a contrasting color along the side of the rattan to indicate the cutting edge. The weapon is "blunt" to limit serious damage being inflicted onto fighters (although being hit hard enough with what is essentially a club can hurt anyway).
Regular combat blades (called 'live steel' by US re-enactment groups) are basically blunt steel blades. In the UK the standard is to have a 2mm edge, Australia has 1.6mm edge and the majority in the US use 1.6mm. As the edge of the blade is thicker than a historical weapon they tend to be slightly heavier than their historical counterparts. Modern reenactment blade makers are quite good at balancing their weapons for use, but the quality varies.
Semi-sharps are blades that have edges much thinner than regular combat blades and tend to be used in training for historical swordsmanship.
Sharps are relatively rare and are primarily used for living history displays or test cutting.
Unfortunately the cost of a weapon does not necessarily indicate its quality. It is a good idea to speak to someone who has purchased a blade from a particular company before purchasing one.
The Role of Blades in the SCA
In one of her essays in Tournaments Illuminated while serving as Steward of the Society, Mistress Hillary of Serendip described non-combat sport blades carried by SCA participants as "steel jewellery". This is an insightful observation; the swords, daggers and so forth worn with our garb, even if they are of functional quality, will never be used for their original purpose. They are, in their game-play or social function, just as ornamental as a bracelet or coronet, and their integral splendour through craftsmanship and adornment can and will vary just as much as the quality of necklaces or buckles vary.
Other blades worn with garb may be equally ornamental but may serve far more useful purposes than "steel jewellery"; examples include the whittling or eating knife and the camp axe.