There was a time -- a time more dignified than our own -- when commonly-used epithets were not invariably vulgar. And thus I come in praise of "blockhead."
As is the case with most things under the sun, one can do no better than to start with Samuel Johnson. And, sure enough, he is (in my humble opinion) the place to start when it comes to the use of "blockhead" as an insult. First, The Great Cham's definition: "A stupid fellow; a dolt; a man without parts." "Part" is in turn defined by him as follows: "[In the plural.] Qualities; powers; faculties; or accomplishments." Clear enough.
Now, let us (briefly) see Johnson in action. In Boswell's Life, he is reported to have said of the poet Charles Churchill: "No, Sir, I called the fellow a blockhead at first, and I will call him a blockhead still." (Boswell, Life of Johnson (edited by George Birkbeck Hill), Volume I, page 35.) In an amusing footnote to Johnson's comment, Birkbeck Hill states: "See post, ii, 173, where Johnson called Fielding a blockhead." (It is at times like this when one realizes what a treasure George Birkbeck Hill is: did anything about Johnson escape his notice?)
But please note this interesting observation by "Miss Reynolds": "his dislike of any one seldom prompted him to say much more than that the fellow is a blockhead, a poor creature, or some such epithet." (Birkbeck Hill, Johnsonian Miscellanies, Volume II, page 270, footnote 6, emphases in original text.) This suggests that Johnson did not often resort to vulgar insults.
As a worthy successor to Johnson in the nineteenth century, I give you John Ruskin: "When, in the close of my lecture on landscape last year at Oxford, I spoke of stationary clouds as distinguished from passing ones, some blockheads wrote to the papers to say that clouds never were stationary." The Storm-Cloud of the Nineteenth Century, The Works of John Ruskin: Library Edition, Volume XXXIV, page 11. Needless to say, Ruskin then went on to prove those "blockheads" wrong. (I will save that story for another time. Interestingly, George Birkbeck Hill and a passage from Homer figure in Ruskin's successful rebuttal of the "blockheads.")
So, the next time that you are provoked by someone, may I respectfully suggest that you consider the use of "blockhead." You shall be in good company.