Using platform-level RNGs for key derivation with hash functions like BLAKE3?
Would it be safe to use an architecture-level instruction like RDRAND or RDSEED (x86) or RNDR (ARM) to generate this key material, and then derive a key that way?

Would you need to derive the same key twice (for example, you're using the key to encrypt data from Alice to Bob, so both Alice and Bob need to derive the key; or if you're using the encrypt stored data, so you need to derive the key both when you store the data and when you retrieve it)?
If so, then RDRAND won't give the same data twice, hence you won't derive the same key.
If not, that is, you are generating the key only once (and storing it if you need to refer to it later), then it should be safe.  On the other hand, there may be little reason to bother with a key derivation function at all; just invoke RDRAND, and use that as your key...