Ending your current insurance policy and adding yourself to your spouse's insurance will create a new policy, as your spouse's insurance will change from an individual to a family policy. Any of your medical conditions diagnosed previous to the start date of the new policy will be considered a pre-existing condition, though your spouse may be exempt from this occurrence due to the new policy being under their original insurance carrier. Simply put, yes, it is likely that the insurance company will consider conditions such as high blood pressure to be pre-existing.
Having a pre-existing condition may not be the major cause for concern that it was in the past. Under the laws of the Affordable Care Act any insurance plan started after January 1, 2014 cannot charge a higher premium nor refuse coverage for any pre-existing conditions. The only exemption to this law is if the insurance is considered a grandfathered policy. A grandfathered policy is simply a plan or policy that was in effect before March 23, 2010. Your spouse's insurance representative will be able to tell you if their employer's insurance plan is grandfathered.
I recommend contacting your spouse's insurance and inquiring if it is a grandfathered insurance plan. They will also be able to assist you with any questions regarding the specific insurance plan your spouse's employer has. You can find more detailed information about insurance laws through the following web links: