I have two questions. Fristly, why is it incorrect to speak of a melting point?
Is it because the impurities in a sample will have various melting points and therefore you should use the term melting range instead?

And secondly, what will happen if the melting block is heated too rapidly? What effect would this have on the observed melting point?

The problem is I don't understand a thing here... the melting point of what? What melting block? I'm afraid that you gave too little information, or that I'm not familiar with the terms you used. :(

It is not acceptable to say melting point because there are start of meltiong and the end of melting. Start of melting is the first trace of liquefied crystal of the sample while the end of melting is the last trace of crystal form of the sample which become liquid. That's wAt I know about that matter.

Hope it is useful;)