When there is a single product group in an order, the app shows whatever rates have been set-up for that group. Blended Rates don't show.

When there are multiple product groups in an order, the app will show whatever Blended Rates are configured.
In order for a blended rate to show, all product groups in an order must have a preferred rate set for that blended rate.

Additional Blended Rates and Preferred Rates explained with an example.

We have three product groups:

The Default group, which contains the majority of products shipping from New York, has 3 rates from UPS:

The Vendor A product group, which contains Sauces from a dropship vendor shipping from California, has 3 rates from FedEx:

The Vendor B product group, which has a T-Shirt from a POD provider, has one rate set-up:

Here are multiple Blended Rates set-up:

Let's take a look inside the 2 Day Shipping Blended Rate

When there is only one product group in an order

The rate will not show, because a blended rate only shows when there are multiple product groups in an order
Instead, whatever rates that have been set-up for that group will show.

When there are products from Default and Vendor A in an order

The rate will show, because a blended rate shows when there are multiple product groups in an order, and each product group as a preferred rate set.
In this case, this rate will add together the UPS 2nd Day Air rate for the Default group, with the FedEx 2Day rate for the Vendor A group, it will appear at checkout as 2nd Day.

When there are products from Default, Vendor A and Vendor B in an order.

The rate will not show, because a blended rate will only show if each product group in an order has a preferred rate set for the blended rate.
In this case, because Vendor B has no preferred rate set, the 2 Day Shipping rate will not appear.