What does mean "REFERENCES" section should be block?
Based on the updated question, I'm more inclined to think that the reviewer is asking you to adopt a certain style to the actual references. I think you need to find out what that is. Perhaps some aspect of the references need to be in all caps. But I imagine once you know the style that applies which should be clear from either looking up author instructions or looking at previous publications.

My original guess:
I think a block font can sometimes refer to an upper case font. My guess is that the style guide in operation requires that your references heading  be written as "REFERENCES" rather than "References". Thus, I imagine the reviewer is simply asking you to conform to this stylistic requirement. 

More broadly, make sure you know which style guide applies when submitting your work and endeavour to conform to its requirements.