A good quality exterior self-priming paint like Dulux Weathershield would be suitable for painting Besser blocks. If you are using the blocks for a retaining wall then it would be an excellent idea to paint the side facing what is being retained with a water-proofing membrane similar to Crommelin 15L Pro-400 WP Roof And Wall Waterproof Membrane. This will prevent moisture entering the rear side of the block, migrating through it, and then"pushing" the paint off the face.
Please let me know if you need further assistance or have any questions.
That looks fantastic and great colour choice! I like how modular Besser blocks are for making plant stands. If you want more room just add more blocks.
Many thanks for sharing your finished project with the community.