Kasbah, are you referring to this type of idea...
http://www.all4naturalhealth.com/blood-type-b-diet.html
even in this article it kinda says the opposite of what you say (referring to type B's and lactose), basically saying it's fine for type B's but in moderation except for those that are lactose intolernat, which to me kinda throws the whole theory into the water.
I don't know how valid this type of idea is...I'm type B, I had severe lactose intolerance (twice in my life), used lactaid products until I discovered that taking a good probiotic which aids with lactose intolerance issues works just fine for me.
I couldn't find anything from the mayo clinic or any other reliable source regarding eating right for your blood type though so I'm on the fence regarding the theory, not syaing it isn't true, just like to have reliable evidence regarding it.
:)