repro1:
Create a New Animator Controller
Create a new Blend Tree
Go into the Blend Tree
(make sure you have no parameters)
Right-click the Blend Tree and select "Add Blend Tree"
(you should now have two chained Blend Trees, that both use Blend parameter)
Create a new Float parameter and name it "A"
Select first Blend Tree and set "A" as it's Parameter.

Deleting the only existing parameter "A" does not reset the Blend Tree parameter to "Blend". It still has an "A" slider that is no longer connected to a parameter. Deleting a parameter that is being used by a Blend Tree, should reset the slider to Blend.
Also notice that "A" no longer exist in the Blend Tree parameter pull-down list, but is assigned as a slider, that does nothing.

In 4.6 blend tree was updated correctly.

repro2:
Create Animator Controller
Create a Blend Tree
Create a second Blend Tree from the first one
Create a parameter
Select the SECOND Blend Tree and assign the parameter
Error: "parameter name does not exist."