Ch and ph would be digraphs, but sk would be a blend. The distinction depends on whether or not the two letters create an entirely new sound (the way "ph" makes the "f" sound, as in phonics), or if they blend together, retaining their root sound (like the "gr" in grape).
Therefore, the word blend has two blends, but the word digraph has a blend and a digraph.
Kn or ck (as in knock) are a bit unique, in that one of the two letters is silent. Nonethless, these are still considered to be consonant digraphs.