Blanket Purchase orders will not show as encumbrance because they are essentially a blue print or template for an actual purchase order. You may create regular purchase orders from a Blanket purchase order. To see a blanket purchase order affect encumbrance, generate a regular purchase order. 

To generate a regular purchase order, follow these steps:

1. Select Record > Purchase Orders 
2. Open the desired Blanket Purchase Order
3. On the top bar click Purchase Order > Generate Regular Purchase Order...
Note: You will not see this option

Note: The line item(s) in a Purchase Order will reflect the encumbrance 
For more information, review How to use Regular, Blanket and Template Purchase Orders in Accounts Payable