I wouldn't use a razor blade - easy to scratch the heatsink creating a heat trap. Use a plastic tool instead. One of the plastic knives that McDonalds gives with their large breakfast items works like a charm. Or a credit card. Remove any remaining adhesive/goop with lighter fluid (naphtha) or acetone followed by a rubbing alcohol rinse. Those lens tissues or Kimwipes (get from lab supply company) are great for cleaning without lint.
.bh.