From what I have read... the blade itself is hot if something actually makes contact with it... melting doors, cotterizing wounds and stuff like that... but if the blade is hot enough to melt through blast doors then the Jedi would burn their hands off from holding it so close to the blade... thats simple thermal dynamics... So the blade cant emit heat... but retains the heat to be released only on contact...
Just my educated guess...
in the Shadows of the Empire, after Luke builds his lightsaber, he takes it out and turns it on. When he does this, he hold his hand right near the blade and he feels no heat. Then he says something along the lines of the lightsaber being built correctly because it emits no heat. So it must only give off heat when it touches an object. Hope this clears that up