Late goals by Antonio Valencia and substitute Ashley Young sent United home with all three points after being made to work for their win by a resolute Blackburn side.
United are now clear favourites to take their 20th title after making it seven wins on the bounce.
But there were a few scary moments for United, Junior Hoilett, Marcus Olsson and Grant Hanley all brought great saves out of De Gea before the visitors struck.
Fergsuon said: "The progress is obvious, everyone can see it.
"He's growing in stature and confidence. We've always known he had great ability. He made three great first-half saves but that's the sort of threat that Blackburn offered.
“They're dangerous counter-attacking."