What is the word for multiple letters that behave like a single sound?

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.

They're called digraphs -- consonant digraphs in your specific case.

More generally, a phonogram is a group of letters (or a single letter) that makes a single sound. For example "igh" in high, "eigh" in sleigh, or "ough" in bought. [Wikipedia]

I believe English has at most 4 letters/sound.

Summary:

• Digraph = 2 letters combo with one sound
• Trigraph = 3 letters combo with one sound
• Quadgraph = 4 letters combo with one sound

.....

• Phonogram = 1-4 letter combo with one sound