This depends on the culture.
For example, If you live in Beverly hills. Riding a Ferrari is normal there. So you don't have to hide your Mercedes.
But if you live in a poor country. I would suggest you to hide your coke.
In other words, Be wise !!!
As for me, I don't like to go and tell people about what I am doing unless I am dont with it. Example would be doing a small project. I will tell others about this project when it is completed. Why would I tell people about my project and at the end it will turn out to be a failure ?
Doing that I have followed everything, I have hidden the blessing. When I am done. I showed it to others because allah love to see his blessings on his slave
and I am basically following this hadith:
Seek the help for the success or victory of your needs by being quiet. For verily, everything that has a blessing in it is envied.
This hadeeth was classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 3/436, hadeeth no. 1453; and in Saheeh al-Jaami’, no. 943.
Also, Scholars have said that the best thing to hide is your engagement until you have agreed with your wife's parents.
and thy used this evidence to come up with this rule:
[Of these stories mention] when Joseph said to his father, "O my father, indeed I have seen [in a dream] eleven stars and the sun and the moon; I saw them prostrating to me."
Yusuf, Ayah 4
He said, "O my son, do not relate your vision to your brothers or they will contrive against you a plan. Indeed Satan, to man, is a manifest enemy.
Yusuf, Ayah 5
They also used this verse as an evidence:
And he said, "O my sons, do not enter from one gate but enter from different gates; and I cannot avail you against [the decree of] Allah at all. The decision is only for Allah ; upon Him I have relied, and upon Him let those who would rely [indeed] rely."
But Take a note at the end of the verse, I will qoute it below:
The decision is only for Allah ; upon Him I have relied, and upon Him let those who would rely [indeed] rely.
No matter what you do, "hide or not hide". You should always rely on allah and know the decision is only for him.
To answer the 2nd part of your question:
"What's the dividing line between being self-admiring and showing blessings? If someone says I'm clever should I say "thank you" or should I say "No I'm not smart" or should I say "Alhamdulillah Allah made me clever"?"
First: Always praise allah for giving you this blessing. BE grateful
So [Solomon] smiled, amused at her speech, and said, "My Lord, enable me to be grateful for Your favor which You have bestowed upon me and upon my parents and to do righteousness of which You approve. And admit me by Your mercy into [the ranks of] Your righteous servants."
Second: Thank this guy for giving you a nice complaint
The Prophet (peace_be_upon_him) said:
He who does not thank Allah does not thank people.
Personal advice, It seems that this thing of "hiding blessing" is somehow making you worried. If this is true then stop right away. This is the shaytan whispering to you to make your life miserable.
In Islam we have these solutions to help you in this problem.
Stop thinking about it, do an activity that will help you forget about it
Tawakul on Allah
Read morning and night Atkhar, You can find it in this amazing booklet, Muslim Fortress. It contains authentic athkar from the prophet PBUH
Read The last three Surahs after the each prayer of the 5 daily prayers.
Make you dua'a that allah will help you remove this fear and whispers of shaytan
After that inshallah you will be fine.