Technically fruits – peppers come in a range of varieties. They also come in a rainbow of colours – each one having a slightly different nutritional makeup.
Most of the time you’ll want to use the sweeter bell peppers in your smoothie recipes. These are missing the compound “capsaicin”, which puts the “hot” into hot peppers.
When blending – you can leave the internal ribs and seeds on. However, they tend to be bitter – so I remove them.
Bell peppers blend well, are pretty subtle in taste and comparatively sweet. This makes them a good source of natural sweetness.
They are part of the nightshade family – so not for everyone. This is especially true as they are being used raw here. There is a great article here that covers the basics.