After you have calculated the average result of the Rating control, you can send it to the server using jquery to consume a POST service that will later update the two custom fields of the Blog Posts with the data from the service (which will be the VotesCount and the VotesSum).
Please check this blogpost for information on how to consume the service using jquery:
Also, in order to update the custom fields you can use the DataExtensions.SetValue method:
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