Have a look at this document.

Set the device message system properties such as the contentType to application/json and contentEncoding to UTF-8.

In the case when your device is using the MQTT protocol directly, use the topic for passing the system properties, for example:

 devices/mydeviceId/messages/events/$.ct=application%2Fjson&$.ce=utf-8

Update:

The following screen snippets are showing an example of the quick test for Data export to the blob storage.

1. Creating a IOT Central Application with a PnP device template (environmentalSensor) assigned to the device1 and Data Export to the blob storage:

1. Using an Azure IoT Hub Tester for simulation of the PnP device1 connected to the IoT Central directly via MQTT protocol:

As you can see, the published topic is

devices/device1/messages/events/$.ct=application%2Fjson&$.ce=utf-8

and 6 messages have been sent to the IoT Central.

1. Showing a telemetry data on the IoT Central dashboard.

1. Using the Azure Storage Explorer for exploring a blob content:

As you can see, the body of each device message is not Base64 encoded json text.

Note, that only contentType and contentEncoding of the device message will decide if the body is Base64 encoded or not.