What is the definition of "Timhon Leivav"?
Rashi: It means a confounding of the heart (mental confusion).
RS"R Hirsh: It is a lack of clarity - obscurity of the senses, thoughts and imagination.
Ha'amek Davar: It means that some people will see and understand the evil, but due to bluntness of heart, which is like a stone, they are incapable of doing anything.
What is the meaning of the first half of the Pasuk?
Targum Onkelos: It means literaly 'You will grope in the middle of the day (when there is plenty of sunlight) like a blind man gropes in darkness'.
Targum Yonasan: It means that you will seek advice to ease your problems ... but there is no-one to give you advice that is correct.