Given is on gae using tipfy (python) the following model:
greeting.avatar = db.Blob(avatar)
What is the template-tag to display a blob (here image)?
In this case the blob is an an image, which is great. Just use images.get_serving_url(blob_key) and you're happy. I've used this function, and trust me, it is awesome for serving images. Just append =sxx to the url where xx is the pixel size you want. It automatically resizes the image and serves it.
If the blob isn't an image, then you're out of luck w.r.t an easy way out. Maybe use the BlobReader to make a string representation and output it?
If it isn't an image or text, though, what could you possibly want to write to HTML?