A digraph is when more than one letter have a single sound...like /ea/ in bread or /ch/ in chat. Digraphs can come at the front, middle, or end of word and can be consonants or vowels.
Blends are different because they have more than one letter but you can hear their sounds and are not one single sound. It is contrary to its name of 'blend'.
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The key thing is that the two letters are commonly linked when printed.