Without forced induction, engines tend not to knock as readily at higher altitudes because the intake charge is less dense. That is why 93 octand gas is sold in Ohio (600' asl), 91 octane gas in Denver, CO (5-6000' asl) and 87 octane gase is sold in Keystone, CO (10,000' asl).
With forced induction, you have to consider what the "boost" really is. A 10 psig (psi gauge) boost setting means that the manifold pressure is 10 psi higher than the atmospheric pressure. At sea level, 10 psi of boost means a total manifold pressure of 14.7 psi plus 10 psi or 24.7 psia (psi absolute). In Denver, CO, where normal atmospheric pressure about 12 psi, 10 psi boost means a total manifold pressure of only 22 psia.
So if all else is equal, the same engine with the same boost settings at sea level and in Colorado, the tendency to knock should be less in Colorado because the intake charge is less dense (less real pressure so charge is less dense).
In Colorado, we need 2.7 psi of boost to just be at the same intake charge density as an engine with 0 boost at sea level. And that's what I mean about a forced induction engine can compensate for the thinner air by increasing boost. Normally aspirated engines just have to live with it.