Those who didn’t vote thinking that it was so obvious we’d stay that it wasn’t worth their time certainly can be blamed. (And it is highly unlikely anyone who wanted to leave would not have voted, so they are in the 17.4M.)
However there will be those who abstained because they couldn’t make up their minds, whether deep thinkers who see the advantages and disadvantages of both with no clear winner, or people confused by the media circus and unable to make any sense of it. These people can’t be blamed for not saying ‘stay’.
Meanwhile, unfortunately there are no statistics for the people in the 17.4M who either:
a) voted ‘leave’ to make a protest, never actually wanting to leave, instead wanting to reduce what they perceived the Govt thought was a strong pro-EU majority, with the sole intention of shaking Govt out of its complacency, but certainly neither expecting nor wanting the result to be leave, and seriously regret their vote, wishing they could reverse it.The figures in terms of the people I know personally who voted leave are 100% didn’t actually want to leave, 0% did want to leave.
b) voted ‘leave’ on the basis of what they were led to believe then through the various high profile media stories, but who, now that the lies have been debunked and the hard reality of what Brexit will mean to them, their families and their future is clearer, think differently and no longer wish to leave, and wish they could reverse their vote. I have no indication of any proportion to which this might apply, but suspect it is a high number.