Add a physical device with Blade chassis checked.
You can either choose an existing hardware model for the device or add a new one using the lookup icon next to hardware input field.
Once you save this, you would be able to add blade servers to this chassis.

When adding a new device, choose type as blade and click "save and continue editing" to go to next screen.

Here you can choose/add the hardware model (type blade) and choose which chassis the blade server belongs to. Later in this article, we would discuss how to do this in bulk using excel and API calls to save you these multiple steps.

Devices > All devices would show location for the blade server as the blade chassis name - Slot #.

From the device chassis view page, you can see all the modules in that chassis by clicking on the "modules list" as highlighted above.

We introduced better web based rack layouts in v4.2.0 for blade visualization. You can view list of all the blades/modules in a chassis when you hover over the device in the rack layout view.

If you head over to Tools > Import > Devices, you can download the sample excel file. You can either edit existing devices or add new devices using this. To focus on blade server/chassis relationship we have kind of hidden other columns here.
As you can see in the image, in first row, we are adding a device type "physical" with "Is it modular Chassis?" . Then in subsequent rows we are adding blade servers as Type "blade" and choosing the Blade Host and the slot #.

You can also create/edit the blade chassis and servers using RESTful APIs in device42. All the arguments are listed on device42 documentation site under device create/update APIs doc.