He has just been expounding on the greatness of Dora to his wonderful friend Agnes, whom he grew up with, to an extent, and we readers have all figured out by now that he's infatuated with the wrong girl--and only later does he figure this out, long after he had married and settled down with Dora. As he is walking out the door...oh, well, I should really let Dickens describe it:
...Oh, Agnes, sister of my boyhood, if I had known then,
what I knew long afterwards--
There was a beggar in the street, when I went down;
and as I turned my head towards the window thinking of
her calm seraphic eyes, he made me start by muttering,
as if he were an echo of the morning:
"Blind! Blind! Blind!"
In that one moment, when David was about to make a huge mistake, there was a portent, out of the blue, frightening him out of his stupidity, telling him how blind he was.
It may sound creepy to you, but don't you wish you had a portent, somebody who could hop out of a back alley at you, shouting, "Stupid! Stupid! Stupid!" when you are about to commit a huge mistake, or yelling "Cop! Cop! Cop!" when a policeman's sitting around the bend with a radar gun?
Of course, what did David do with the portent? He ignored it. Only long afterwards did the scene occur to him, far too late to have done anything about it. Perhaps those same portents shout at us, but we ignore them, too, and keep heading headlong into the messes we create for ourselves.
If only we'd listen!