Imo being sacked by Chelsea isn't a blot on your CV, as its pretty much an expected outcome. Our former managers have mostly all gone on to very good jobs quite quickly, and if AVB had stayed at Porto and then for instance taken a job at Real Madrid he would be fired there as well very quickly imo - he learnt a lot in his time here, when he arrived he showed 0 ability to adapt and made unbelievably naive errors of judgement and just would NOT adapt. Playing the ultra high line here is what cost him the job, it was a terrible tactic with our set of players and he absolutely refused to try anything else. If you're that naive and stubborn in any ultra high pay job you should expect to be on your way out. The managers that didn't get a fair shot here, like RDM and Avram Grant, got a massive leg up in their career. No doubt RDM will get another shot at managing in the top flight somewhere at some point, and Grant obviously went on to manage else where.
The key is that if a manager has confidence he is better than the guys that failed here, he looks at the mistakes they made and says "I'm not that naive," then he should be happy to come in here and take a shot at it.