'unblossomed' is formed in English from the negative prefix 'un-' (Old English 'un-', meaning 'not') + 'blossom' (from Old English 'blōstm'/'blostm', meaning 'a flower' or 'a mass of flowers').

'blossom' developed from Old English 'blōstm' into Middle English 'blossom' and then into modern English 'blossom'; the prefix 'un-' has been used since Old English to negate adjectives and past participles, producing compounds like 'unblossomed'.

Originally 'blossom' meant 'a flower' or 'to flower'; the compound 'unblossomed' came to be used to describe something 'not yet flowered' or figuratively 'undeveloped' and retains that meaning in modern usage.