Winger David Rowan was in flying form, capitalising on Lansdowne's numerous handling errors and he used his blistering pace to score two tries in difficult conditions.
Credit must go to the 'Rock forwards, marshalled by lock and captain Des Dillon and flanker Mick Carroll, who did not buckle under tough Lansdowne pressure for most of this third round tie.
However, the 'Rock defence may have been slightly disappointed to concede a 79th minute consolation try when replacement Craig Ronaldson got in behind the line of defence to score Lansdowne's only try.
Speaking to IrishRugby.ie after the match, Blackrock College coach Barry Gibney explained the importance of this victory after last week's AIB League loss to Clontarf last week.
"There's always a point to prove. Last week in the league was tough, I was disappointed with our performance in the second half and you've just got to pick yourselves up and get back to winning ways," he said.
"It's always about how you react and what you learn from the defeats that make you really a better team, so we've just got to look forward and keep improving our performance.
"This cup match was just about keeping the opposition on the back foot but I thought we played well in patches - the half-backs (James Power and David Madigan) controlled the game well.
"It was important to have your defence organised and make the hits and try to play a bit of territory and control the ball when you do have it," added Gibney.
Due to a Leinster 'A' fixture on Wednesday evening, Gibney and his coaching squad were slightly hampered with selection issues and they had to field a much-changed side without the likes of in-form players David Moore and Niall Morris.
"With the Leinster 'A' game during the week, we didn't really have a big say in selection so there were places to be filled within the squad.
"But I thought the guys who stepped up took their opportunities and that's what it's all about really."
'Rock will now face either Garryowen or St. Mary's College at the quarter-final stage of the AIB Cup and although he was happy with the overall display, Gibney admitted that his side will need to eradicate the mistakes going into the next round.
"There were plenty of mistakes out there. Our discipline cost us quite a number of penalties, that can catch us out sometimes so we have plenty to work on as well."
Meanwhile, Lansdowne head coach Willie Clancy conceded that it was always going to be tough going into battle with a Division 1A side.
"I think Blackrock are the in-form team in club rugby, they're an outstanding side and you could see the gulf between Division 1A and Division 2," he told IrishRugby.ie.
"Playing in a competition like the AIB Cup gives us the honour of playing against a powerhouse like Blackrock, but it was always going to be tough to beat a team like that."
Despite the defeat, Clancy insisted that his side now need to bring their focus back to Division 2 and with a tough local derby against Bective Rangers next week, they will need to be in good shape to continue their unbeaten run in the league.
"The AIB Cup was a bonus but it's over now, so our main priority is always Division 2 and we're trying to achieve our goals there."