If Blokchain.com or a third party would manipulate the outcome of this API than almost a third of all on-chain transactions could be tampered with.
That's an interesting thought...
In theory, somebody controlling the feerate recommendations could collude with multiple miners and share the additional fee revenue. Thousands of transactions paying a higher feerate would affect other feerate estimators, for example, Bitcoin Core's estimator as well. This would increase the potential revenue even more.