Cyanobacteria resemble green algae because they were found in moist and damp places and were capable of photosynthesis-preparing their own food. They contain the photosynthetic pigments chlorophyll (green) and phycocyanin (blue). Hence, they appear bluish green in colour.
Hence, they are known as blue green algae.
However, cyanobacteria are prokaryotes which lack cell organelles as found in the algal cells.