If you wash your hands and your mind goes blank and you dont remember if you said a bracha or not. do you make a new one? Do you wash again and then make a new one?
I would think that if you know for sure that your hands are now tahor, then there is no problem. You washed your hands in order to purify them, so even if you didn't say a bracha, your hands would still be tahor. The tahara is taluy on the netila, not on the bracha.
In my opinion, I would think that someone should be nervous about washing one's hands and saying a bracha on the netila could be a bracha levatala. What if your hands are still tahor since the last time you were notel them? Then what happens is that you are notel them again and you say a new bracha - that would be a bracha sh'eino tzricha!
The general rule with berachot is safek berachot l'hakel- in a case where we are not sure whether we make a beracha we err to the side of not making another one. This is a subset of the general rule of safek d'rabanan l'kulah (we are lenient about doubts which occur on mitzvos from the Rabbis.)
The exceptions are berachot that are d'orayta "from the Torah" which are bentching (grace after meals) and birchat hatorah. However, one would be covered by the bracha of ahava rabba before shema in the morning if one was not sure about birchat hatorah and would not say it if one has said or plans to say that bracha.
Therefore you one would not go back and wash or make a new bracha.