Permanent Damage from Sunburn While on Accutane?

Caution should be exercised while on Accutane.  Accutane works on the pilosebaceous unit and decreases the body's ability to epithelialize a wound during treatment. Avoid deep sunburns and trauma while on accutane. 

Your description does not sound like it will lead to permanent scarring. Obviously you need to take great care to avoid the sun while you have the sunburn, but if you take care of yourself and remain out of the sun for the next week or so, you should heal fine. In the future you need to be more diligent about covering yourself while outside.
Use A minimum 30 sunscreen applied multiple times daily when outside.
Use full body coverage for your clothes, linen, cotton and other light fabrics can help with the heat. Wear a hat and long pants when outside fo rextended periods.
Try to limit outdoor periods to the beginning or end of the day, when the sun is not at it's peak.

Sunburns are common while on accutane but rarely cause any permanent damage.  Use a good cortisone lotion several times a day to help resolve the sunburn as well as take aspirin or advil for the anti inflammatory effects and pain relief and see your doctor. 

Both Accutane and oral antibiotics for acne can make you more prone to have sunburn. Consistent daily sun protection and reapplying high SPF sunscreen every 2 hours between 10am-4pm when outdoors are critical. You may experience prolonged post-inflammatory redness or pigmentation after a sunburn which will slowly fade with frequent emollient application and adherence to daily sun protection. Unfortunately, every sunburn makes you more prone to develop skin cancers such as melanoma. Therefore, all fair skin persons need to have a "head-to-toe" skin check by a board-certified dermatologist every year.

Accutane can make your skin more sensitive to sun light. A second degree sunburn (blisters) usually does not lead to permanent damage. However, there may be a risk of darker pigmentation. See your dermatologist for an evaluation.