I think we're getting a little off track from Blu's questions! Also, I'm pretty sure Blu is over 6, so he's good to go.
Did you find out high high 4-H jumps are? I would recommend that you eventually get Blu comfortably jumping things a few inches taller than the 4-H limit so that he can easily handle the jumps at shows.
Someone mentioned 4-H probably goes up to 1'6" and Blu can definitely do that! YOU could probably run and jump over that yourself. It's pretty small. Blu is 16.1hh right? So is my horse and he gets lazy and pretty much walks over anything too small. He really perks up and has fun when we're at 1 foot or more. You could try free-lunging him over jumps. Then you can easily increase the fence height since you're already on the ground. Of course, he'll be able to jump much easier (and higher) without a rider. Then you can take some cute pictures too!
Take Jazzy's advice and listen to Blu...when he's totally comfortable with a height, increase it a little. Slow & Steady wins the race. Luvs2ride also has good advice about conditioning with caveletti, hills, and pattern work. When he's strong on the flat, it'll be easier for him to jump. And easier on you (when you're strong too). We have this little pond at our barn that dries up in the summer. It's perfect for riding up and down the edge for hill conditioning. We don't have any trails near by. Anyways, I even noticed that when we're riding up the hill, I naturally get in a very comfortable forward position and move well with my horse. Of course, when I jump with him in the arena, my legs slip back, I'm too ahead of the motion, I don't release enough, etc etc. I think I need to ride that hill more often!!
Another thing to remember, jumping is more than just getting over an obstacle! Even at a 6" crossrail, you can add things like flower boxes, tarps, or other scary objects (near by). Then Blu will be ready for anything at a show plus he'll be less likely to get bored if you're sticking to smaller heights for a while. Also, set up little courses in your mind and ride them. Concentrate on your preparation before the jump (coming in straight, centered, and balanced) and after the jump (balanced, correct lead, looking where you're going).
Wow, I'm really rambling! Anyways, have FUN! That's the most important part. Have you jumped before with other horses?