It seems to be a small underestimation. A more precise estimate would be more than 5%, around 6%.
It turns out that people study the prevalence of hair color to understand which genes are related to it -- this gives us access to prevalence data for different hair colors, and -- in the US -- this seems to be around 10-12% for blonde hair of Americans of European descent.
Han J, Kraft P, Nan H, Guo Q, Chen C, et al. (2008) A Genome-Wide Association Study Identifies Novel Alleles Associated with Hair Color and Skin Pigmentation. PLoS Genet 4(5): e1000074. doi:10.1371/journal.pgen.1000074
The study needs these numbers to be representative to exclude systematic bias in the choice of people with different hair colors, but they refer to American of European descent.
This, and the observation that the different studies give very similar numbers, leads me to conclude that a believable estimate (relative to European-descent Americans) lies between 10.7% and 12.7%.
Now, of course there are other ethnicities in the US. Interestingly in large part they are basically completely non-blonde. Thus we can correct the numbers and get a minimum estimate easily.
From this list we can see that there were 148,471,345 US nationals of European descent in 2011. This means between 15.9 and 18.9 million blonde Americans. Given that there were a total of 310 million Americans, we can estimate that in percentage, blondes are between 5.1% and 6.0%.