Another explanation of 'seeing it is hidden.' The angel said to him: 'I am not able to tell you my name for the Holy One, Blessed be He, gives us a name to accord with the errand on which He sends us.' Thus 'Seeing it is hidden' means: in accordance with each particular wonder which He performs through us He gives us a different name.
From the parashah's description of the laws of the Nazirite, the midrash takes us on a journey involving stories of that most famous Nazirite, Samson. The midrash cited above imagines the scene described in Judges 13 in which an angel visits Samson's parents to tell them that, after years of infertility, they will miraculously conceive. Samson's father asks the angel his name, to which the angel replies: "Why do you ask my name, seeing it is hidden?" (Judg. 13:18).
The word "hidden" in Hebrew here is peli, which usually means wonderful, extraordinary. The angel's name is hidden in the sense that the extraordinary nature of his work makes it unnoticeable. By extension, the extraordinary nature of our own work is hidden from us. It takes a special set of lenses to walk through life hearing not only the whisperings of angels guiding us, but seeing in our daily tasks a call, a sense of divinely given purpose. Each of us has an errand to run from On High; that errand is reflected somehow in our very title, our names, that which we are given or (to paraphrase the wonderful poem by Zelda) one that we acquire for ourselves. God performs wonders through us, even if we do not feel "called," even when the meaning of our actions is hidden.