M.L.
Obgyns aren't specifically trained for ailments such as herniated discs. They don't typically have xray machines, or may not have the contacts to give you a referral to specialists for issues such as those. A familiy Dr can review your issue and refer you to a neurosurgeon or physical therapy, but that is not a n ob's area of training.
You should still have a family Dr for those other things who has a broad knowledge and referrals for other health issues. As for blood tests, I suppose you could always call your ob to see what they cover. Also. I don't know how your insurance works, but mine views ob visits as specialist so co-pays are more expensive. Insurance may not approve non women's health related issues through an ob specialist.