The nice thing about randomness is that given a random blob, you can xor a non random blob & it's just as random. So this is one input of entropy amongst a cocktail of sources
That’s true statically. Keeping that true with changes over time is HARD. For example, it matters that the non random blob never gets to see either the random input or the output.
Actually that's the property of xor, not randomness. Data xor uniform distribution -> results in uniform distribution. It doesn't have to be random.