If someone wanted to get into older FPS games, I wouldn't recommend Blood. I would go with Duke Nukem 3D which is designed more fairly and consistently. The difficulty ramps up every episode to the fourth, and it never is too cheap with you. Blood is more a case of concentrating the good stuff towards the beginning some people associate with shareware. I like episode 2 the best. I know some people would really hate that, but I wish someone would go and give me more weapons and ammo in ep 1.
I would also recommend Quake. The original game has great level and enemy design. It's also a very fairly designed game where enemy attacks have tells. Unforseen features like strafe jumping and rocket jumping also allow you some versatility in how to tackle levels. Each of the four episodes was mostly handled by a different level designer, so they all feel differently thematically/graphically. It doesn't have great bosses, and only two of them, and that was already a weak point of the genre. Very active modding community too. Some mods have entirely new campaigns and boast new graphics/weapons, such as Age of Enmity and Arcade Dimensions.