For itchy skin, with nothing on the outside, consider two skin conditions ~ artificial urticaria and pruritus.
Artificial urticaria, when the skin is itchy there is nothing wrong with the appearance, but after scratching there will be a wind ball, the shape of the wind ball is consistent with the traces of scratching. After a while, the clamminess will subside. Positive skin scratch test.
Pruritus, which is simply itchy skin and does not appear as a windburn when scratched, with a negative skin scratch test.
The causes of both artificial urticaria and itchy skin are complex, so only symptomatic treatment can be given to stop the itchy skin and try to find the cause in order to break the root cause. There is no single test for itchy skin. There are many items that can be checked for itchy skin, unlike liver and kidney disease, which is considered to be bad for the kidneys, checking the urine, checking the kidney function, and doing an ultrasound. There are so many factors associated with itchy skin, allergies, high blood sugar, hyperthyroidism, abnormal liver and kidney function, tumors, psychological factors and so on may cause itchy skin. Often the cause cannot be found.
So with itchy skin, it is important to first diagnose what skin condition it is and then try to find the cause.

Chinese medicine talks about deficiency of yin and blood, blood deficiency and wind movement. Try taking Six-flavored Di Huang Pill orally.

First of all, you can consider hives, which is more in line with your description, and then the blood test is normal that depends on the items you check your blood for!