If my mostly black text document has some blue text throughout document that I would like to auto mass change from blue to green text.
If you have used a Character Style for the blue text then it is simply a matter of changing the Character Style and all text using that style will change, assuming no direct formatting
If you have used Direct Formatting rather than styles you might be able to change the colour using Find and Replace but it rather depends on consistent use of one colour of blue.
Identify the colour of the font that you are looking for from an example in your document and note the name or Hex code.
Click Edit > Find and Replace (Ctrl+H), in the dialog box that opens click once in the Find field then click the Format button near the bottom of the dialog. In the Search for formatting dialog click on the tab Font Effects. In the field labelled Font colour click the drop down arrow and select the colour that matches the one you identified. Click OK
Click once in the Replace field then click the Format button. In the Search for formatting dialog click on the tab Font Effects. In the field labelled Font colour click the drop down arrow and select the colour that you want as the replacement. Click OK.
You are returned to the Find and Replace dialog, click Replace All. Repeat for another shade of blue if inconsistent use of blue.
Ideally, in the Replace dialog you could replace with a character style but that doesn’t seem to be available yet.
In addition to @EarnestAl.
Direct formatting often lead to inconsistencies in documents.
I recommend you learn how to apply style sheets:
Professional text composition with Writer and
[English documentation](https://documentation.libreoffice.org/en/english-documentation/)