Hormones mix with venous blood .why not oxygenated?

There are three types of chemical messengers

1. Acting on same cell
2. Acting on neighbouring cells
3. Acting on distant organ/tissue

Hormones are from the third type. They act on distant organ/tissue.

Now coming to the hormone release into blood,

Arterial blood come towards to secreting gland. And venous blood leaves the gland.

Veinous blood will come back to heart and it will get redistributed. Eventually it will reach the target organ.

There is no need of hormones for the secreting gland, so doesn't get secreted into arteries. As arterial supply will bring back hormones to the same gland which may cause some metabolism.

While venous secretion results into supply of hormone to the target tissue. Hormones reach the target tissue/organ via arterial route.