Thank you for contacting Syncfusion support
We can achieve your requirement in our public event called RenderDayCell of the Calendar component in the Blazor platform. We have logged the requested requirement as the feature at our end and this support will be included in our Essential studio 2019 Volume - 4 release. We appreciate your patience until then.
You can track the status of the requested feature in the below feedback link from below.
Regards, Saranya D