Q: yosemite error installation

Reboot, to go recovery mode,

OS X: About OS X Recovery - Apple Support

go to Disk Utilities, select the HD you wish to install 10.10 on and click Repair Permissions.

What make/model mac do you have and what are the specs? If you meet the requirements and the above does not work I would try deleting the copy you have and downloading it again to see if there was a corruption.

"Couldn't bless the installation disk: /" sounds like a job for the Holy Father in Rome but aside from being full of surprises I don't think he's Apple Certified.

While you are booted into the Recovery Partition, also run repair disk.

Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support

see 2nd paragraph: Try Disk Utility

then with the disk still selected and you've done the step by step in the paragraph click repair permissions