There are many mobile apps and websites that can blur or obscure faces in images/photos (check the list on WikiHow). Built-in image editor tools such as MS Paint (for Windows) and Paintbrush (for Mac) can be used for simple editing.
There are various mobile apps and video editor software that offer blurring/obscuring functions. If you park your videos on YouTube, you can use YouTube Studio to blur your videos.
Participants should be advised not to identify themselves. Likewise, transcripts should identify participants by code rather than by name.
Image by StockSnap from Pixabay
A recording can never be completely anonymous. Blurring out faces does not guarantee protection.
A person can be identified from visual details (e.g. scars, tattoos), a distinct clothing, name on the screen, a landmark in the background, as well as audio details such as a distinctive voice or accent.
Tips for recording (source: Witness Library):