United began brightly and fashioned a decent opportunity in the 13th minute when Jordan Stewart crossed dangerously and Henri Camara arrived to rifle a left-footed shot over the bar from 12 yards out.
Moments later, Preston conjured their first goal threat when striker Jon Parkin was given the time and space to drill in a right-footed effort from 20 yards out.
The pendulum swung back in the Blades' favour in the 15th minute when matchwinner Ward tried his luck from distance and Neill Collins made a fine block to avert the danger.
The home side looked the more dangerous team and they came close again in the 19th minute.
Derek Geary flighted in a teasing cross and Ched Evans arrived to plant a firm header goalwards, but Sean St Ledger, back at Preston after his proposed transfer to Middlesbrough dramatically collapsed, showed his quality with a fine block.
St Ledger joined Boro on a three-month loan in September - prior to Gordon Strachan's arrival at the club - and was set to complete a £4.5million switch to the North-East in the January transfer window.
However, although the player has been a regular in Middlesbrough's first team, he has now returned and his future lies at Deepdale.
Camara then flashed a left-foot shot just wide in the 23rd minute as United sought to make their dominance pay.
Yet for all their territorial advantage, Blackwell's men struggled to create genuine goalscoring opportunities.
Shortly before the half-hour mark, referee Trevor Kettle was forced off through what looked like a hamstring injury and was replaced by fourth official Nigel Murphy.
United continued to look the more likely to score as half-time approached. Geary again did well to find Evans and he volleyed straight at North End goalkeeper Andy Lonergan from 12 yards.
Camara then twice failed to hit the target when he should have done better and United were almost punished for their profligacy when Preston defender Billy Jones crossed to Chris Brown and he headed goalwards, only to be denied by a superb save from Blades goalkeeper Mark Bunn.
In the 63rd minute, Preston midfielder Chris Sedgwick collected possession and hit a shot from 25 yards which flew over the crossbar.
Blackwell made a double substitution in the final 20 minutes, bringing on Glen Little and Lee Williamson for Camara and Stephen Quinn.
In the 74th minute, Richard Cresswell saw his header blocked by Jones but striker Stephen Elliott then headed wide for the visitors.
The match looked destined to end all square until Ward hit a free-kick from 30 yards out which flew into the bottom corner of Lonergan's net.