The quantity that has been converted to sales order is shown in 'Make order qty.' field in the blanket order header.
When you click on the 'Make order qty', you can drill down to the sales orders that were created from this blanket order.
The above method is from the end-user point of view. From the database level, the Blanket Order No. and Blanket Order Line No. will be copied to the Sales Order Line. This info can be seen from Sales Order line subform as well.