Good question. I had to brush off the dust on my Biochem book for this one. It sounds like the two nitrogens that are part of the imidazole ring on histidine's side chain give the amino acid both acidic and basic properties. The basic nitrogen (with the free electrons) can abstract a proton from a donor group, only to have it shuttled and relayed to another group via its more acidic nitrogen.
It's this amphoteric property that makes it an important AA in the active site of enzymes, and explains why it's considered a basic amino acid -- even though its pKa would suggest otherwise.
Well at some level it isn't going to acidic because it doesn't have a proton to donate. There are also factors which can raise the pKa like neighboring side chains. Still regardless it will be slightly basic/protonated due to the not vast difference in pH to pKa.