The win, coupled with draws for the Seattle Sounders and Sporting Kansas City on Sunday, gave the Rapids the Western Conference title, the No. 1 seed and a first-round bye in the upcoming postseason.
LAFC, fighting for a playoff spot, threw everything it could at Colorado but finished a disappointing 12-13-9 (45 points), failing to make the league playoffs for the first time in its four-year franchise history.
Colorado (17-7-10, 61 points) also got goals from Collen Warner, the veteran's first goal since March 2017, second-half substitution Cole Bassett and another late sub, Dominique Badji. The Rapids won their first conference regular season title in franchise history.
The deficit forced LAFC head coach Bob Bradley to get his top striker, Carlos Vela, into the game seemingly earlier than he hoped. But it didn't make a big difference.
Lewis, off a turnover, took a touch and rolled in his second goal in the 53rd minute. LAFC, forced to attack because they needed a win and help from other teams to get into the playoffs, appeared finished at that point.
But Lewis took his time coming off the field after the goal and the Rapids weren't quite ready with a substitute. LAFC took advantage of the imbalance and made it 3-1 with Cristian Arango's 55th-minute goal.
Bassett picked his spot and fired from outside the box past Blackman in the 63rd minute, making it 4-1.
Brian Rodriguez scored for LAFC, which still had life down 4-2 in the 71st minute. But Badji, subbed in only two minutes prior, finished off the momentous win for the home side with a 79th-minute goal.