I have severe asthma and high blood pressure. would a black widow bite be more severe in my case?

Yes: The black widow bite is a stress to your whole system and would certainly exacerbate existing conditions of poor health.

Black widow bite: Individuals struggling with serious illnesses are more likely to have a poor outcome from Black widow spider bites. Here is an excellent NIH article: http://www.nlm.nih.gov/medlineplus/ency/article/002858.htm