The letters ue are not the most common way to spell the long /oo/ sound. Unfortunately, there is no rule for when the letters ue make the long /oo/ sound within words. However, it is usually used to spell the long /oo/ sound at the end of words, such as blue or due. Two exceptions I have found at gruel and cruel. Students will have to orthographically map the spellings of words with the long /oo/ sound. Luckily, most kids only need one to four exposures to a word to orthographically map it for reading. Spelling will likely take more practice.
PRACTICING THE long /OO/ SOUND WITH THE LETTERS ue
You can check out my video with a routine for practicing mapping and spelling words where the letters ue are making the long /oo/ sound.
Here is the word list I promised! Click on the picture, and you will get to the .pdf.