I think it would be best to blanch them. No more than a minute. They will keep their colour better. They may not be suitable for use as a stand-alone vegetable afterwards, though. I sometimes do it with the leftover carrots at the bottom of the fridge but only use them in stews and soups on defrosting.
I’ve mashed carrots with swede in the past and frozen it successfully.
Freezing as soon as possible after picking, or digging up, is meant to mean much higher vitamin levels are retained.
When we visited my aunt in Canada she asked us to take her to a farmers market (new thing to us at the time). She bought two big bags of veg, Rinsed off any mud then shoved them all into the freezer. She lived well into her 90s so can’t have done her any harm.