Not too late... thanks to the late bloomers!
I just checked High Park's update on the state of their cherry trees and the Toronto park predicts the full bloom for May 5-10. (It usually occurs around the last week of April.)
(Update as of May 2, 2016: Full bloom expected between coming weekend May 6-7 or the next)
Two years ago, I showed up on the first week of May... and missed it, but there were lots of petals on the ground and the crowd had deserted the place. High Park in the spring is always gorgeous anyway so I wasn't too disappointed but I took a mental note to return the following year.
Last year, thanks to the update we can get on High Park's website, I saw the cherry trees in full bloom and I could take photos of this glorious moment to create the HIGH PARK Cherry Blossom Stroll in my guide Toronto Urban Strolls 2.
I showed up on a Friday and never would have thought so many Torontonians had it in them to "play hookie"!