If you are talking about the color, you can say 蓝色 (lan2 se4). The word 色 means color, so saying this phrase refers to only the color.
- 他最喜欢蓝色 (ta1 zui4 xi3 huan1 lan2 se4): he likes blue the most
If you are talking about a blue _________, using blue as an adjective, you would simply say蓝 (lan2), or蓝的 (lan2 de1)
- 今天的天空好蓝 (jin1 tian1 de1 tian1 hao3 lan2): Today’s sky is so blue
- 这是蓝的天空 (zhe4 shi4 lan2 de1 tian1 kong1): This is a blue sky