This was a RTFM...

http://world.episerver.com/Documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-60/Page-Providers/

You need to ad a property to search over all providers

PropertyCriteriaCollection criterias = new PropertyCriteriaCollection()
            {
                new PropertyCriteria
                {
                    Name = "PageTypeID",
                    Type = PropertyDataType.PageType,
                    Condition = CompareCondition.Equal,
                    Value = Settings.Instance.BlogPostPageTypeID.ToString(),
                    Required = true
                },
                new PropertyCriteria
                {
                    Name = "EPI:MultipleSearch",
                    Value = "*"
                }
            };