Over the two legs Barca couldnt beat them (home or away), and in the end luck is a side arguement. Maybe Barca played "more attractive football", maybe they had more shots on goal, maybe they had lots of possesions of the ball (and then did very little decisive with it, given for much of the tie they only faced 10 men). But you could also say Barca were lucky as they got a very soft penalty. In Barca's really big matches in the CL this season they have beaten Milan twice (group stage and knockout), drawn with Milan twice (very first game and knockout away leg)) and lost and drawn with Chelsea. They are not an all-consuming, unstoppable force that some claim there are. Milan showed how it's possible to contain the attacking threat of Barca, Chelsea built on this, and also their system and players are very suited to counter-attacking.
People keep knocking Chelsea but they deserve some kudos, look at the teams they beat Valencia (eliminated in the group stage), Napoli (on paper perhaps the most difficult Italian team in the competition, one that had eliminated City, Villareal and taken Bayern to the wire), Benfica (on paper perhaps not world class but still a team that had eliminated Man Utd and also the Russian Champions) and Barca ("darlings" of football and a team some alledge to be the finest sporting side in the history of world sport) and fainally Bayern Munich playing in effect at home, with one of the best home records in Europe. Chelsea can only beat the teams that were put in front of them and they deserve credit for this.