Blissful means perfectly happy. A blissful holiday, blissful ignorance, blissfully unaware.
It is at the top of the happiness range and slightly literary, which is why it is used ironically as often as not. Blissful ignorance and blissfully unaware are the fixed phrases and both are gently mocking. Note bliss; and that the Old English blithe, meaning happy, is a related word — and bliss was influenced by bless, which is separate.
1adjectiveblissfulPerfectly happy.
“We spent a blissful week on a quiet Greek island, swimming and reading.”
synonyms: idyllic, delightful
blissful is a quality of great happiness.
Middle English blisful is bliss plus -ful, from Old English blis, joy, related to blithe. Bless is a separate word that influenced it.
Middle English
blisful
English, today
blissful
From Middle English blisful, bislvol, equivalent to bliss + ful.
Etymology from Wiktionary, CC BY-SA