The Seasiders took a dream lead after just five minutes at Stamford Bridge as Jake Daniels steered home from Dannen Francis’ cross.
The hosts hit back though and held a 2-1 half-time lead after an evenly matched first 45 minutes.
Hopes of a cup upset were raised when Luke Mariette’s driving run into the box ended up with the ball being diverted into the back of the net via an unknowing Chelsea player.
But it was the home side who secured their spot in the final four, where they will face either Blackburn Rovers or Nottingham Forest, after converting a penalty 20 minutes from time.
A healthy number of parents, family, friends and supporters made the journey down to the capital to cheer on the young Seasiders.
Blackpool were given an early warning sign inside the opening 40 seconds, as Derrick Abu was set free down the right.
The full-back delivered a dangerous cross into the six-yard box which Will Squires did well to turn behind, somehow managing to avoid conceding an own goal in the process.
Blackpool soon settled down though, forming a solid, organised shape that saw them come alive whenever they won the ball back on the counter-attack.
Unbelievably, the young Seasiders took the lead just five minutes in.
It was a counter that set Dannen Francis free down the right, the winger doing brilliantly to pull the ball back across the face of goal for Jake Daniels to sweep home - sending the Pool fans crazy.
Two minutes later, Daniels turned provider as he teed up Joe Strawn, who shot straight at goalkeeper Prince Adegoke.
Francis continued to cause the Chelsea left-back all sorts of problems with his pace and tricky, as he danced his way into the box before seeing another dangerous
With John Terry watching on, the Blues drew level after 14 minutes with a goal out of nothing.
It was a real moment of quality from Harvey Vale, who dropped his shoulder to create a yard of space before curling a beauty into the top corner from long range.
When Chelsea did threaten, more often than not it came via Abu down their right flank, who was a constant threat.
On one occasion, he shifted the ball into Vale in the centre, who had the presence of mind to pick out Silko Thomas in space, but fortunately for the Seasiders he could only fire wide from point blank range.
From this point onwards, Chelsea took charge of proceedings as Blackpool were left chasing shadows.
While they kept in a tight shape, when they did get the ball they struggled to keep hold of it, resulting in sustained pressure from the home side.
McLachlan was forced into his first real save 10 minutes before the break when he got down at his near post to tip Charlie Webster’s low shot behind.
Blackpool came so close to retaking the lead six minutes before the interval when Joe Strawn was set free down the middle - but Chelsea keeper Prince Adegoke made an excellent save to thwart him.
McLachlan almost went one better at the other end, making two super hero saves to deny striker Malik Mothersille only for the Chelsea striker to force the ball over the line at the third attempt to put the hosts in front.
It didn’t deter Blackpool though, as they ended the half strongly and were the better side at the start of the second-half too.
They fashioned a good chance for last round’s hero Arnold Matshazi, who was slipped through only to be denied by a last-ditch challenge eight yards out.
Unsurprisingly, Chelsea still offered a big threat and they caused Pool’s backline problems with the pace of Mothersille, who slipped the ball back into the path of Ronnie Stutter, but Jack Moore got across to make a crucial interception.
It proved to be a vital bit of defending from the Blackpool right-back as the Seasiders would draw themselves level moments later.
The goal owed a lot to the impressive Luke Mariette, who drove forward into the box before seeing his pullback glance into the back of the net via Chelsea defender Luke Badley-Morgan.
The game was delayed for a good 10 minutes or so due to a couple of stoppages, as both McLachlan and skipper Michael Fitzgerald required treatment.
For Fitzgerald, it proved to be his final action of the game as he limped off to be replaced by Johnson Opawole.
McLachlan, meanwhile, was forced to face a penalty when Chelsea were awarded a pretty blatant spot kick as Will Squires brought down Abu in the box.
Charlie Webster made no mistake, sending McLachlan the wrong way before bizarrely celebrating in front of the away fans housed in the corner of Stamford Bridge.
Any chances of a late Blackpool revival looked unlikely as players continued to go down complaining of cramp.
Instead, Chelsea were able to keep hold of the ball and make the Seasiders do all the running - which was the last thing they wanted to do in the circumstances.
Saying that, Pool did enjoy a late break when they had men free either side of Strawn, who opted to shoot only to see his low effort saved by Adegoke.
The Seasiders again came within inches of taking the game to extra-time when Adegoke somehow managed to turn Daniels’ powerful drive around the post.
The game ended on a slight sour note for the Seasiders as Jack Moore was shown a second yellow card and was sent off. The defender looked absolutely distraught as he headed town the tunnel, burying his head in his jersey.
But Moore and his Blackpool teammates can rightly be proud of themselves for delivering a proud, gutsy display where they took Chelsea all the way.
It’s a cup run to be proud of.
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