[C] flower, esp of plants admired chiefly for their flowers (eg roses, tulips, chrysanthemums) 花（尤指主要供观赏的, 如玫瑰、 郁金香、 菊花）: These roses have beautiful blooms. 这些玫瑰花开得真美. =>illus at App 1 见附录1之插图, page ii. Cf 参看 blossom.
[U] (fig 比喻) freshness; perfection 新鲜; 完美: be in/have lost the bloom of youth 正值[失去]青春.
[U] covering of fine powder that forms on ripe plums, grapes, etc 粉衣, 粉霜（成熟的李子、 葡萄等表面所生的一层霜粉）.
(idm 习语) in (full) bloom (of plants, gardens, etc) flowering （指植物、 园圃等）开花: The garden looks lovely when the roses are in bloom. 玫瑰花开时园内美丽诱人. * (fig 比喻) Her genius was in full bloom, ie at its best or highest point. 她才华横溢（处於最佳状态或颠峰时期）. take the bloom off sth cause sth to lose its freshness or perfection 使某事物失去新鲜感或失去光彩: Their frequent rows took the bloom off their marriage. 他们经常吵架, 使得美满婚姻恶化.
> bloom v
1 [I] (a) produce flowers; flower; blossom 开花: Daffodils and crocuses bloom in the spring. 水仙花和番红花在春天开放. (b) (fig 比喻) flourish; prosper 茂盛; 繁荣: Our friendship is blooming. 我们之间情长谊深.
2 [I, Ipr] ~ (with sth) (a) (of a garden, etc) be full of plants or flowers in bloom （指花园等）长满茂盛的花木: The garden is blooming with spring flowers. 园内春花盛开. (b) (fig 比喻) be in a healthy or flourishing condition (because of sth) （由於某种原因）处於健康或隆盛状态: They were blooming with health and happiness. 他们既健康又快乐.
而flower的意思相对比较简单，你和上面的对比一下便知。
虽然也有实例中类似bloom的灵活用法（例如，作为动词，或者in full flower-in full bloom)，但是总体来说那是不多见的、非主流的用法。
日常用语中单指花朵时，flower更为常用，bloom则较为正式。
参考资料：灵格斯词典