I have been bitten by what appears to be a brown recluse spider. the bite is on or very near the veins on my wrist. can this be serious?

See derm: Yes, it usually will turn black and spread. See your dermatologist for treatment and evaluation.

Potentially: The brown recluse (loxoceles reclusa) can cause a variety of symptoms after a bite, however, a lot of the time, the bite is minor. If the area is becoming painful, oozing, blistering, or necrotic (black dead tissue), or if you have any systemic signs like nausea, vomiting, fevers, muscle pain or other problems, then you should see a physician asap.