Thanks to @Lichtsignaal and Mr Roman from Openelec forum.
To install Blynk Server in a Raspberry Pi running Openelec follow this steps:
1- Download Putty and FileZilla.
2- Download the Java Development Kit HardFT.
3- Using, for example, 7zip, extract the JDK and you’ll need the “jre” folder.
4- Alternatively you can just download the “jre” folder I extracted from Java Version 8 Update 73 from my google drive.
5- Open Filezilla, go to “File”, “Site Manager” and enter the RasPi network ip, select “SFTP” as Protocol, Logon Tipe “Normal”, User “root” (by default) and Password “openelec” (also by default).
6- Connect to RasPi by SFTP and in root directory (storage) create a folder named “java”.
7- Put the “jre” folder inside the “java” folder.
8- Open Putty and connect to the RasPi. Introduce the following commands:
cd ~
touch .profile
echo "export PATH=$PATH:/storage/java/jre/bin" >> .profile
chmod +x /storage/java/jre/bin/java
reboot
9- After restarting you need to reconnect Putty and execute the following:
java -version
10- If all went OK you should get the java version installed.
11- Download the last Blynk Server java executable.
12- Copy the server “jar” file to /storage/java/ using Filezilla again.
13- Now, to make it autostart use Putty to enter:
cd /storage/.config
touch autostart.sh
chmod +x autostart.sh
14- Use nano or open autostart.sh in wordpad trough Filezilla and copy the following code:
/storage/java/jre/bin/java -jar /storage/java/server-0.13.4.jar -dataFolder /home/pi/Blynk &
Enjoy!