>These are some causes :
-Dry skin. Dry skin is a common cause for itchiness, especially in areas where the skin is most delicate.
-Excessive moisture. Moisture from sweating can be irritating. Anal itching can also be caused by fecal incontinence and frequent diarrhea.
* Excessive washing. Wiping and excessive scrubbing can cause anal itching.
- Chemical irritants. Soaps, colognes, douches and birth control products contain chemicals that can irritate skin in and around your anus. Scented or colored toilet paper can be irritating to people with sensitive skin.
- Food irritants. Anal itching may be the result of irritating chemicals present in certain foods and condiments, such as chili peppers or hot sauce.
- Medications. Anal itch may occur as a side effect of certain medications such as antibiotics
-Overuse of laxatives. Improper use of laxatives can lead to chronic diarrhea, anal irritation and itching.
-Hemorrhoids. Anal itching can be a symptom of hemorrhoids. However, most hemorrhoids don't itch.
- Anal fissures. These tiny tears that occur in the anus for a number of reason may cause itchiness.
- Infections. Sexually transmitted diseases can cause anal itching. Parasites can also cause persistent itching around the anus. but you have taken vermox had you been diagnosed with it ?
-Thrush. Fungal infections in the genital area can cause anal itch.
-Skin disorders. Skin conditions including psoriasis, seborrhea and eczema can irritate the area in and around your anus, causing anal itch.
- Other causes. Anal itching may also be caused by ANXIETY AND STRESS
>>These are tips to be done to decrease or treat this feeling especially that your stool test is normal:
A lot can be done to naturally maintain and promote anal skin health. It is a good idea to support overall systemic balance by eating lots of fresh fruit and vegetables and whole grain foods, ensuring plenty of fresh air, adequate sleep, moderate sunshine and regular exercise. Also avoid tight clothing and synthetic underwear.
>Add some oatmeal to your bath water. Oatmeal has naturally soothing qualities that help soften and heal irritated and itchy skin.
>Promote tissue healing with Aloe Vera. Aloe Vera mixed with olive oil or tea tree oil can be applied to the area to promote healing.
>Keep the anal area clean. Moistened toilet paper or wipes are gentle on the skin and make it easier to thoroughly cleanse the anal region.
> Keep the anal area dry. Unscented baby powder or cornstarch can be applied to the area to keep moisture at bay and prevent chafing and irritation.
> Eat plenty of fiber. A common cause of anal itch is constipation. Increasing your fiber intake will promote healthy digestion and stools and possibly relieve symptoms and prevent future occurrences. Fruits, vegetables and whole grains are great sources of dietary fiber.
> Wash undergarments with a mild soap. Laundry detergents can be extremely irritating due to the chemicals present in some of them. The same goes for fabric softeners. Wash underwear with a mild unscented soap to prevent future irritation and itch.
> If thrush or candida is suspected,there are several antifungal creams that can be used
Since your blood tests are normal look then for other causes such the one above such as anxiety or stress or certain allergy treating these might help beside following the tips mentioned in my answer .