"Bloom" infers that the flower is at its fullest, brightest stage.
"Blossom" can be a verb describing the process of a bud slowly opening up into a bloom, or it can be a noun which describes a flower which is in the early stages of "opening" up into full bloom.
If you like blooming flowers, it means you appreciate the beauty of flowers at their brightest and best. After they bloom, they will slowly begin to wilt and die.
If you like blossoming flowers, you enjoy watching the process of buds slowly opening and spreading their petals, eventually reaching full bloom.
That distinction is too neat, I fear.
Let me start with the Oxford English Dictionary (OED) definition:
bloom [verb] -
produce flowers; be in flower
blossom [verb] -
(of a tree or bush) produce flowers or masses of flowers
So both mean to produce flowers and blossom is used paticularly for trees and bushes which produce masses of flowers ("
the apple tree is blossoming") - 'blossom' [mass noun] itself can mean a mass of flowers ('
cherry blossom').
Bloom is more used for cultivated flowers* which are often bigger and showier than the blossoms of fruit trees and shrubs ("
The chamelias have started to bloom"). That said, I think most people would use 'blossom' interchageably in this sense ("
The chamelias have started to blossom"). And also, for a lot of cultivated flowers (eg roses) there exist wild varieties - I don't think we make a distinction in saying one blooms but the other blossoms!
Now look back and you'll see that 'bloom' - and not 'blossom' - has an additional meaning: 'be in flower'. So when flowers have fully opened up and their colours are on full display they are no longer blossoming; they are blooming.
In short, when flowers are opening up, it is almost always fine to say 'blossom', 'bloom' only sometimes; when flowers are on full display, it is always fine to say 'bloom', never right to say 'blossom'. At least according to the OED.
But that distinction only applies when using the verbs; when we use the mass nouns 'bloom' and 'blossom' (usually when we say flowers are 'in bloom' or 'in blossom') the distinction completely disappears.
Again, from the OED:
bloom -
the state or period of flowering
blossom -
the state or period of flowering
Identical! Note here that, unlike the verb 'blossom', 'in blossom' can refer to the state of flowering, in other words it can be used to describe flowers when they are on full display. Here are the two examples from the OED:
"the apple trees were in bloom"
"the fruit trees were in blossom"
What's the difference?
Still, even though the OED doesn't distinguish, I personally would be more likely to say 'in blossom' for the flowers of trees, bushes and shrubs, and 'in bloom' for cultivated garden flowers. Actually, I'd also be more likely to say 'in bloom' for wild flowers as well, especially if they were colourful or big ("the bluebells are in full bloom").
* 'bloom' [countable noun - not often used] in fact means "a flower, especially one cultivated for its beauty"
