Hello Al and congratulations, your new garden is off to a flying start.
Those blotches are caused by leaf miners, you may even find then burrowing in between the outer sheaths and sucking the sap. Very frustrating and not much remedy I know of, some watering might help and the beetroot can still grow, also you can compost those leaves at harvest time. Miners do the same to shard and leaf beet, mostly on medium to larger leaves.
I have my beets netted against sparrows, their leaves are popular.
