Hi, once again apologies for the late reply.
I'm not sure that the grey glass blob at the base of the vase exists. I think the shadow at the base of the vase is caused by wear. To my eye (and I don't have particularly very good vision anymore so may be wrong) the vase appears to be monochrome. If I get a chance I'll try to rephotograph it and post the new images. Looking down through the vase from the top there appears to be no change in colour.
I'll continue to research ZB when I get a chance as well as the Scandi makers and report back when I can.
Thanks to all for taking the time out to comment! :-)