blindyoudaoicibaDictYouDict
blind: [OE] The connotations of the ultimate ancestor of blind, Indo-European *bhlendhos, seem to have been not so much ‘sightlessness’ as ‘confusion’ and ‘obscurity’. The notion of someone wandering around in actual or mental darkness, not knowing where to go, naturally progressed to the ‘inability to see’. Related words that fit this pattern are blunder, possibly from Old Norse blunda ‘shut one’s eyes’, blunt, and maybe also blend.

By the time the word entered Old English, as blind, it already meant ‘sightless’, but ancestral associations of darkness and obscurity were retained (Pepys in his diary, for instance, writes of a ‘little blind [that is, dark] bed-chamber’ 1666), and traces of them remain in such usages as ‘blind entrance’.

=> blend, blunder, blunt
blunderyoudaoicibaDictYouDict
blunder: [14] When blunder first entered the language, it meant ‘stumble around blindly, bumping into things’, which gives a clue to its possible ultimate connection with blind. Its probable source was Old Norse blundra ‘shut one’s eyes’, forerunner of Swedish blunda and Norwegian blunda (Jon Blund is the Swedish equivalent of ‘the sandman’), and very likely a descendant of Indo-European *bhlendhos, from which blind comes. The first record of the modern sense ‘foolish mistake’ comes in Edward Phillips’s The new world of English words 1706.
=> blind
bluntyoudaoicibaDictYouDict
blunt: [12] Blunt originally meant ‘dull, obtuse, foolish’ in English, and it has been speculated that behind it there lay an earlier ‘dull of sight’, linking the word with blind. A possible source would be a derivative of Old Norse blunda ‘shut one’s eyes’ (whence probably also blunder). The application of blunt to dull, non-sharp edges or blades developed in the 14th century.
=> blind, blunder
blunder (v.)youdaoicibaDictYouDict
mid-14c., "to stumble about blindly," from a Scandinavian source akin to Old Norse blundra "shut one's eyes," from PIE root *bhlendh- (see blind). Meaning "make a stupid mistake" is first recorded 1711. Related: Blundered; blundering.