Krishna is above all these three qualities. Sattva, Rajas and Tamas affect only the residents of Samsara. As he says in the Bhagavad Gita 18.40:
na tad asti pṛthivyāṁ vā
divi deveṣu vā punaḥ |
sattvaṁ prakṛti-jair muktaṁ
yad ebhiḥ syāt tribhir guṇaiḥ ||
There is no being existing, either here or among the demigods in the higher planetary systems, which is freed from these three modes born of material nature.
So, since Krishna/Vishnu resides in the spiritual world, he is not affected by them at all. His body is also not affected by such Gunas. He is completely pure. Now, this pure does not indicate Sattva, as a Sattvic life also has parts of Rajas and Tamas. Since he is above all the three, he is completely pure.