A further use of 'bloom' is the really archaic term for a chunk of freshly smelted iron out of a bloomery (not used except by enthusiasts and historians, since bloomeries haven't been widely used for hundreds of years).

Blossom usually refers to the flowers of fruit or nut trees - almond, cherry, apple - and is usually a mass noun. The cherry blossom was early this year. Bloom is a count noun referring to individual flowers, typically of large decorative flowers such as roses or tulips. Flowers like this are usually priced per bloom. In both the above case, the noun can also be an abstract noun which describes the flowering state: a tree is 'in blossom' if it is currently flowering. Bloom is alsp a mass or count noun referring to growths of microorganisms such as algae or yeasts. Literally, the verbs 'to bloom' or 'to blossom' refer to the flowering of the corresponding types of flower, with connotations of joy at seeing such beauty. Metaphorically they are used to describe similarly beautiful transformations, for example when a child becomes a beautiful adult.