Why do you need to learn the cross block?
Of all the blocking techniques, it’s the most useful. Period. From a cross block, as you’ll see in the video, you can block, grab, choke, trap, break, and dislocate limbs. It’s simple and powerful. Make this block part of your self-defense practice. Imitate what you see in the video. As you become more comfortable, increase your response time.
Hapkido involves a lot of grabbing, trapping, tripping, and control (over the unfortunate idiot who decided it was a good idea to attack.) The above video isn’t from a Hapkido school, but I can see that their training is influenced by it, and jiujitsu. Martial arts is an ever-evolving practice. Learn what works and skip the rest (if you’re in it purely for the self-defense aspect.)
As Mr. Myagi said, and I’m paraphrasing, the belt is only good for holding up your pants. Actually, it’s also a great weapon, but we’ll get into that another time. The point is, you can get into martial arts for a lot of reasons, but rank achievement should be at the bottom of the list. There’s always someone who can kick your butt from here to shinola, regardless of the color of your belt.
Practice. Focus. Repeat.
Note: As with every technique I post, 1) you need to practice with a willing adult participant, 2) I don’t endorse any of the sites or teachers, and 3) you need to be aware of and know the self-defense laws in your state. The purpose of these posts is to provide you with information about techniques I would use in certain situations. If you really want to become an expert in self-defense, then you need to find a dojang or dojo. If you have questions about how to identify a good dojang or dojo, post your questions in the comments.