The "historic" designation will have a positive effect on the value of the home making it "rare'.
Due to that fact the comps will be more difficult to acquire within in the typical local comparable neighborhood. The appraiser will most likely take this into consideration and find "like properties" from a broader area and make the necessary adjustments for neighborhood variances.
What happens if the property does not appraise for the selling price depends on the contract. If your realtor has included an appraisal contingency, or in the case of a VA, or FHA loan, the home must appraise for the sales price or the price is typically renegotiated.