This is a bit premature, its only been one season and Barca are still in it. It wouldn't be a surprise if they win it, in which case you would be looking at 6 consecutive CL wins by one of the big two Spanish clubs
Era defined not just by the winners but by the overall performances (sorry for the long post):
2013: Bayern win, Barca lose to Bayern, Juve lose to Bayern. Madrid lose to Dortmund, Atletico not in the competition.
2014: Barca lose to Atletico, Bayern lose to Madrid, Atletico lose to Madrid, Madrid win. Juve fail to qualify for knockout rounds.
2015: Atletico lose to Madrid, Madrid lose to Juve, Bayern lose to Barca, Juve lose to Barca, Barca win.
2016: Juve lose to Bayern, Barca lose to Atletico, Bayern lose to Atletico, Atletico lose to Madrid, Madrid win.
2017: Barca lose to Juve, Bayern lose to Madrid, Atletico lose to Madrid, Juve lose to Madrid, Madrid win.
2018: Juve lose to Madrid, Bayern lose to Madrid, Madrid win. Barca lose to Roma, Atletico don't qualify for the knockout rounds.
2019: Barca still alive, Atletico lose to Juve. Juve lose to Ajax, Bayern lose to Liverpool, Madrid lose to Ajax.
So call these teams the group of five. From 2014-17, a four-CL period,
none of these teams lost a knockout tie to a team outside the group of five, and the only blemish is Juventus not qualifying for the knockout round in 2014. In 2013 and 2018, the boundary years, there are two "blemishes" per year.
In 2019, we've already seen three eliminations of these sides by teams outside the group of five and we're likely to see a fourth.