It seems most (or all?) Blyxas need more time to establish.
I've just realised that my Blyxa is Novoguineensis, not Japonica.
So can't chime in much, but I've noticed many tanks here in Thailand
grow it (Japonica) fine, very fine indeed, at temp around or higher than 28C.
Heck, Blyxa in natural habitat here (don't know what species is
native to here though) grows in very high temp, under blazing sun in
rice paddies and streams.
Growing it with my heater set to ~74-76F but it's getting to 80F inside my apartment as Spring approaches. As said best by others, I think we've all gone through an adjustment period when initially planting B. japonica. It's soon -- give it a couple weeks and judge from new growth.
Bonus cross posted from APC: found trimming (stress?) helps turn it pink. I'm sure this has been mentioned a few times over the years. Notice the flower buds on some stems, too.
Might be easier to appreciate the color against the green Hemianthus and Riccia sp from the top.