Is this a PDF form that was created in an app other than DocHub? If so, each PDF form will have its own default color for the text fields, and it’s not something that DocHub can override. Any forms created in DocHub should always default to black for the Text Fields.
If it is a PDF form created in an app other than DocHub, if you open it using another PDF app, you should find that it will also default to purple for the text color in text fields.
You could add DocHub text fields on top of any fields made by another app by using our Fields Manager so that they default to black:
How do I add fields to a document or template?