The biggest difference between a good blitz and a horrible one is how to rocket grab and following up, i've personally played blitz since beta and usually i consider myself very good on him lol , practicing his rocket grab will help greatly in games , trinity force is by far the best item for blitz , if ur having trouble with health early game get a phage, if not i use to always like sheen because it used to be extremely op with his E ability ... it's still very useful just not as potent
anyways im off track a bit but on blitzcrank just make sure u can aim his Q ability and that u follow up correctly , always power first a target after hitting them and if they're low ulti ... he devastates people with his 1 - 2 - 3 combo , blitzcranks rocket grab is extremely useful when people are running. The best technique i use with blitz and deceiving people with his Q is by walking around a lot (he's a champion that needs to be clicked more than most) and as i turn my back to an opponent rocket grab them , works most of the time because people get a false sense of security when blitz has his back turned , also any chance u can grab someone near a tower try and do it because blitz is extremely effecting at bringing people into tower range stunning and than killing the opponent.
Well i hoped that helped and sorry for the dodgy grammar :P