All roles have some form of buff or other. Generally, it is better to train up your buffs up to the highest cheng permissible given your character's level, as you benefit from the greater boosts sooner than later. The same goes for skills, especially nukes.

Manuals

Training buffs with the help of manuals saves a lot of time. There are three types of manuals:
Beginner - applies up to and including ch3
Intermediate - applies up to and including ch5
Advanced - applies up to and including ch10

Beginner manuals are the cheapest, and advanced manuals the most expensive. Manual prices also depend on the multiplier effect they confer, so (I) is cheaper than (IV).You can obtain manuals that aid in buff training up to ch5 from the clan base, the npc who teaches basic clan kungfu, at nominal prices. So do at least use these manuals, if not better ones, when training your buffs when they are under ch6.

Exp Events

Exp events increase your skill exp rate as well as your (levelling) exp rate. For example, dxp doubles your skill exp rate.

When to train buffs and skills

The effect of manuals compounds with exp events, and premium if any, to give a greatly increased skill exp rate. For example, training on a (IV) manual at 5xp with premium gives
5 x 5 x 1.5 = 37.5
times the base skill exp rate.

We are often tempted to grind during exp events and train buffs during normal exp. I suggest otherwise. Consider again the example above. Now, you train using an (IV) manual with premium at normal exp, giving
5 x 1 x 1.5 = 7.5
Note that you have to use 5 (IV) manuals to achieve the same outcome, compared to 1 during the exp event.

In the first case, you spend 0.5 hours training buffs during 5xp. To make up for lost exp, you grind 2.5 hours during normal exp. In the second case, you spend 2.5 hours training your buffs, and 0.5 hours grinding during 5xp. Both work out to give the same amount of exp and skill exp. But, there is an important difference - the number of manuals used and the cost associated with them. If each advanced (IV) manual costs 1 million, you would have spent an unnecessary 4 million to achieve the same buff level. More than that, 2.5 hours of grinding will likely net you a greater value in drops than 0.5 hours. Assuming you can get the equivalent of 1 BE per hour in terms of value, using the parameters given in this example, you would have wasted more than 5 million just by training at the wrong time.

Blinking HP and Skill Exp

Skill exp rate doubles when your hp is blinking, i.e. your remaining hp is less than 10% of the maximum. A good way to go about this is to acquire sufficient external wounds, so that you can meditate to recover ve without having to face the hassle and associated risk of dying in trying to get your hp down again. Once again, this compounds with any exp boosts you receive from exp events, premium, or manuals.

Does blinking hp boost buff skill exp?

Unfortunately, it doesn't. You get the same buff skill exp rate whether your hp is blinking or otherwise.
It only applies to skills which deal damage, such as nukes, smashes or actives.

Conclusion

Train your buffs during exp events using manuals.
Get your nuke training over and done with by acquiring sufficient ew and training during exp events using manuals.