One bitcoin is, in theory, interchangeable with any other without a loss of value. However, bitcoins with a dubious history of transactions can, and apparently are, being refused in some circumstances. Clearly some bitcoins have less utility, less value, than others.
The traceability of a bitcoin leads to it possibly being rejected in some transactions, not because ant given bitcoin is (again, in theory) no different than any other, but because one bitcoin’s history may be tainted.
The “blood diamond” analogy seems appropriate - such a diamond remains a diamond, and is technically no different from a comparable “clean” diamond, but reputable dealers and customers will avoid them. Effectively rendering these diamonds of less value than others.
In theory bitcoins are fungible. In practice they are not.