Leona is pretty decent if you're already good with her. The only reason I would recommend against her is that you'd probably rather have blitz.
His base stats outrank hers: more hp, ad, armor, mr.
His Q beats her E in almost all cases on Dom since there are few minions to get in the way, even though her CD is much lower.
Her Q is about even with his E.
Her W shield+her passive damage is slightly more tanky, while blitz' MS/AS buff W + his shield passive is slightly more damage and much more movement.
Her Ult is a bit nicer, but he gets his twice as often and it's easier to hit with (his does more base damage, but he probably won't be stacking AP so hers will actually hit harder).
She can certainly defend points better. Initiation is a toss-up since she has more stuns & can take more hits. He can do a lot more damage and get to where the action is a lot more quickly. She could certainly hold bot better if your bot needs to switch. Blitz is pretty useless bot.
So she's certainly viable, especially if you're premade & you can coordinate her passive, but she's a bit underclassed. If you're looking for someone to hold a point & stun, Ali beats here there.