A consonant digraph contains two letters but only makes one sound: the digraph sh makes the sound /sh/. Other common consonant digraphs are sh, ch, wh, th, ck. A blend contains two or three consonants that each make their own sound but blend together to make syllables and words: in the word sleep, the letters s and l form sl. Other common blends are st, fl, sk, and gr. In other words, a digraph corresponds to a single phoneme whereas a blend corresponds to two or three phonemes (sounds) blended together.
For example, the word tree contains a two-consonant blend of t and r. Each letter makes its own sound and these sounds are blended together into tr. The word splash contains a three-consonant blend of s, p, and l. Each letter makes its own sound and these sounds are blended together into spl. Whereas, the sh in ash is a digraph because s and h make a single sound.