I've never heard 'abloom', though I'd understand it and I wouldn't be surprised if it was hiding in an old* dictionary somewhere! 'In bloom' is fine; or 'blooming'. Flowers bloom, but trees and shrubs are 'in blossom' or 'blossoming'. Often, colloquially, the simple word 'out' is used: Be sure to visit when the azaleas are out.
*Some dictionaries - OED for example - are published 'on historical principles'. So even a modern edition contains words that are no longer in use.
BobK said: Flowers bloom, but trees and shrubs are 'in blossom' or 'blossoming'.
Shouldn't "in blossom" be "in bloom"?
Merriam-Webster's Learner's Dictionary
in bloom or in full bloom : having flowers ▪ The bushes should be in bloom [=in flower; flowering; blooming] soon. ▪ These plants are very fragrant when they are in full bloom.
That's an American dictionary; it may have some Br Eng coverage (I don't know). In Br English trees and shrubs have blossom; 'in bloom' is occasionally used more loosely, but it would be very strange to refer to 'apple bloom'.
I'd expect the second only for trees that have a large, well-formed flower. Often, colloquially, people just use prepositions: "out" = in blossom, "over" = finished: 'The magnolia in my front garden was out for 7/8 weeks, but now it's over.'