If somebody has blocked arteries does he probably also have high cholesterol?

Probably: Statistically speaking, it is likely that the LDL cholesterol (the fraction of cholesterol found in the liporporteins associated with atherosclerosis) is elevated. However, roughly 30% of people may not have elevated LDL cholesterol. The atherogenicity of LDL cholesterol depends on both the amount (the number you refer to) and the type, small dense (which is "bad") or large which are less so.

Yes: A strong association between blocked arteries and high cholesterol does exist. Bear in mind that an even stronger association exists between smoking and blocked arteries.