If someone has given you rep once and then given it to you again before they have given it to 5 other people, it will not count as much. That is represented as a gray block. A gray block is represented when someone who has no reputation gives you reputation.
They gray boxes are from new members who don't have enough posts or rep points themselves yet so it shows as neutral. Negative ones will be red and the positive ones are green. Visit the thread mentioned above by stscac for more information regarding the reputation system.