The Dutch word blij [blɛi] means happy, glad, pleased or delighted. It comes from the Middle Dutch blide (happy, cheerful, joyous), from the Old Dutch *blīthi (calm, happy), from the Proto-West Germanic *blīþī (happy), from the Proto-Germanic *blīþiz (serene, mild, pleasant, pleasing, delightful, friendly), from Proto-Indo-European *bʰlī- (light, fine, pleasant) from *bʰleh₁-/*bʰel- (to shine) [source].

Here are some related words and examples of how it’s used (from bab.la and Reverso):

• blijdschap = joy, gladness
• verblijden = to gladden, delight
• blij zijn = to be glad, rejoice, enjoy, be happy
• blij maken = to gladden, cheer up
• heel blij zijn = to burst with joy
• blij zijn met een dode mus = to get all excited about nothing (“to be happy with a dead mouse”)
• Ik ben blij dat je ervan zult genieten = I’m glad you’ll enjoy it
• Ik ben blij je eindelijk te ontmoeten = I’m pleased to finally meet you
• Niet iedereen zal hiermee blij zijn = Not everyone is going to be happy with this

Words from the same root include the Swedish word blid [bliːd] (mild, kind), the Danish word blid [ˈbliðˀ] (gentle) and the word blíður, which means kind, obliging, mild, tender, affable, friendly or good-natured in Icelandic, and hospitable, hearty, friendly, sincere, pleased, mild or smooth in Faroese [source].

The English word blithe [blaɪð / blaɪθ] also comes from the same root, via the Middle English blithe (glad, happy, joyful; gentle, mild; gracious, merciful; bright, shining; beautiful, fair), and the Old English bliþe [ˈbliː.θe/ˈbliː.ðe] (happy, gentle) (to shine) [source].

It means carefree and lighthearted, or very happy or cheerful, and also lacking or showing a lack of due concern, heedless, casual and indifferent [source].

It tends to be used in certain expressions, such as:

• He spoke with blithe ignorance of the true situation.
• She had a blithe disregard for their feelings.

Some related (and rarely-used) words include blitheful (joyous), blitheless (sorrowful, sad, pitiful, miserable, wretched), blithely (without care, concern or consideration; or in a joyful, carefree manner), blithen (to be(come) happy), and blithesome (happy or spriteful, carefree).

Blithe [bləið] is more commonly used in Scottish English and in Scots, and means joyous, cheerful, happy, glad or well-pleased. A related word, used particularly in Orkney and Shetland, is blithemeat, which is a thanksgiving feast after the birth of a child [source].

In Shetland blithe is written blyde and means glad. Here are the Blyde Lasses, a folk duo from Shetland: