Daniel Bogdanovic appeared to have sent the Tykes on their way to a fifth win out of seven league matches when he headed home at the far post in the 74th minute, but Henderson spared Blackwell further embarrassment.
Barnsley fully deserved to edge ahead in the 53rd minute through Anderson De Silva's thunderbolt finish before Henderson levelled with his first penalty on the hour mark.
Barnsley made just one enforced change with skipper Stephen Foster suspended and Rob Kozluk recalled at full-back.
Tykes striker Iain Hume, who fractured his skull in the corresponding fixture last season, was again named on the bench.
Blades manager Kevin Blackwell feared he would be unable to field 11 fit players last week and was also without his captain, former Barnsley favourite Chris Morgan, due to a one-game ban.
Morgan, the player involved in the Hume injury, was ruled out of what would have been a hostile reception after collecting fifth yellow card of the season against Newcastle last Monday.
In Morgan's absence, Blackwell handed a debut to Jon Fortune, a summer signing from Charlton, while striker Jamie Ward returned from a torn hamstring and broken wrist.
United's other centre-back, Matt Kilgallon, superbly denied Bogdanovic with an early block following Anderson De Silva's sweeping pass.
The Brazilian then headed towards the bottom corner, only for United goalkeeper Ian Bennett to save brilliantly and a busy opening saw visiting forward Ched Evans thump a 20-yard drive just wide.
Soon after Bennett dived at full stretch to keep out Bogdanovic's volley.
Lancashire referee Lee Mason had stern words for Jon Macken and Andrew Davies after the latter pair clashed in the 21st minute as a full-blooded derby simmered towards boiling point.
Hugo Colace's volley was spectacularly saved again by Bennett in the 25th minute and only desperate defending denied stand-in Blades skipper Henderson at the far post from Ward's cross and again from Walker's ball in.
Barnsley defender Ryan Shotton was booked after his late challenge left Kilgallon writhing in agony as half-time approached.
Kilgallon was forced off soon after to add to Blackwell's injury woes, with Derek Geary making his first Blades appearance in over a year as a substitute.
Bennett failed to show for United after the half-time break, replaced by Mark Bunn, who had little to do before being beaten by Anderson's angled rocket after the Blades had failed to clear a 53rd-minute corner.
Blades winger Keith Treacey then fired in a low shot against the post as the visitors countered before Macken fluffed a gilt-edged chance to double Barnsley's lead from Dickinson's fine cross.
The visitors were then handed a route back into the match before a crowd of 12,998 had had time to catch breath.
Tykes goalkeeper Luke Steele was adjudged to have hauled down Evans on the six-yard box and Henderson slammed home the subsequent penalty.
Barnsley restored their lead in the 74th minute.
Macken's driven cross from the right was met at the far post by Bogdanovic, who stooped to head home at the far post.
But referee Mason awarded the visitors their second penalty when Hammill upended Walker in the area and Henderson slotted home his seventh goal of the season.