qolbinau, I don't think they are intentionally blocking anyone out of the casting, but I'm sure they had something in mind, at least at the start. And remember that poster here who couldn't wrap their head around the the Jewish character being replaced by a person of color?
Anyway, had to go back a bit to remember where I first heard about the colorblind casting, turns out it was March 2018:
"Stacey Mindich is the producer who commissioned Pasek and Paul to develop the show nine years ago. She told me teenagers would see the show and later return with their parents.
Some had assumed that either Ben Platt, who originated the part and won a Tony; or Taylor Trensch (pictured above, with Laura Dreyfuss as Zoe), who recently replaced him, would play Evan in a West End production.
But Ms Mindich surprised me by saying she had woken one day at 3am with the realisation that ‘Evan has to be played by a British actor’. She also said they would use colour-blind casting, so he could be played by an Afro-Caribbean or Asian actor."
Producer of hit Broadway musical Dear Evan Hansen seeks a British actor to play the title role in London
"Hey little girls, look at all the men in shiny shirts and no wives!" - Jackie Hoffman, Xanadu, 19 Feb 2008