What’s OTG blocking?
You do it by jumping or SJing (I prefer SJing) and immediately doing addf (no pause) lk mk or hk, just like the 2nd half of each set of the ROM infinite. Since you’re new at rushing down I suggest you have an assist that can set up the beginning of the combo after you rush down, i.e.:
tj.hk (that means triangle jump - jump addf) land c.lp + assist s.mk
Note: the c.lp basically eliminates the need for you to dash in after you land, because you often land too far away to do c.hp or c.lk c.hp. c.lp and s.mk have damn good range for normals, which lets you combo in the assist without dashing in.
Now, depending on what assist you have (Psylocke AA, Cyclops AA, Sent proj), the combo will continue differently. If you do c.lp+Psy AA, after she hits you can either walk or dash forward and OTG with c.lk c.hp and start an air combo or OTG with c.lk c.hk. If you SJ cancel the c.hk you can go directly into ROM, or you can cancel c.hk into Hyper Grav, c.hp sj.hp addf sj.hp (which leads to an easy crossup after they wake up), OR c.hp sj.hk addf sj.d+lk sj.mk and start ROMing from there. If you use Cyclops he basically sets up an air combo for you, so dash forward and SJ up to do what you like. If you use Sent-proj, you have to dash forward (don’t dash too early or too late) and c.hp and finish off the 5H (in case you don’t know, the 5H is c.hp sj.hp addf sj.hp sj.hk c.hk), or you can do an air combo/ROM. You can ROM straight off of Sent’s proj assist, but you can get more damage with the c.hp.
Now, you should have at least one assist that can remove the need for dashing, but if they die, it’s easier to dash in after tj.lk tj.mk and do c.lk c.hp rather than tj.hk. However, attacking in the air with an air combo finisher (either an hp or an hk) after launching will give the flying screen effect if the opponent was hit multiple times in a jump-in (like the tj.lk tj.mk). This means that if you try tj.lk tj.mk land dash c.lk c.hp sj.hk to start the ROM infinite, you won’t be able to because the hk will send them flying.
You can set up lots of resets and unmashables with the ROM, but it’s easier to do the 5H for now. Does 62 damage on average defense (lifebar has 143 points), which is almost 50%. Toss in Sent proj and it goes over 50%.
To block rushdowns, you kinda have to guess what’s coming. Practice against good rushers to get accustomed to the possibilities of attack.
EDIT: You can practice the [tj.lk tj.hk] infinite (repeat brackets) to practice triangle jumping.