BP readings probably are high during flare ups, as they would be with any stress. But it's how they are flexible and balance out that counts.
Generally my BP readings are fine, but I am aware that when my body is put under stress, they will naturally rise.
BP is like an elastic band, It's meant to be flexible, but it's meant to return to its natural shape. It's only when it consistently doesn't, that there is a high BP reading as a general thing.
There are a few things you can do to naturally help high BP. One is Lime flower tea (Could be called Linden flower or Lindenblumen) I don't think that would have a negative effect on IBS. Raw garlic probably would, though that's good for regulating blood pressure too.
Meditation could help?
I would say keep in contact with your doctor if you are always getting high BP readings. And of course let them know if you are trying any natural or herbal remedies.