If any scenario that 'blacklists' some bitcoins is implemented (IMHO it won't, but if we're discussing it..), then any bitcoins 'tumbled together' with blacklisted bitcoins would be blacklisted as well.
Which would mean that either people would stop using tumbling services (since the process would turn 'good' bitcoins into 'tainted' bitcoins as soon as a single participant wanted to anonymize stolen BTC), or tumbling services would disallow tumbling any blacklisted bitcoins, or both.
Which would mean that either people would stop using tumbling services (since the process would turn 'good' bitcoins into 'tainted' bitcoins as soon as a single participant wanted to anonymize stolen BTC), or tumbling services would disallow tumbling any blacklisted bitcoins, or both.