SO.... You power up you're PS4 and it just sits there with a flashing blue light
until you power it off...
Some times this is fixible but be prepaired to except it as non fixible.
The constant pulsing blue light means the console's built in pre boot diagnostics failed hardware checks and so stopped the boot process.
Finding the fault is rarely possible without extensive searching and diagnostics with various tools by a technician. The time involved is vast, and 60% of the time just not worth taking on as a viable fix.
In many cases its a complete failure of a non replaceable chip such as APU or NOR chips. Said chips are coded to the motherboard and not transferable. Replacing the entire group of primary coded chips is lengthily and expensive in resources and may be done only to find motherboard has other failed smd micro components that have to be found.
The NOR chip which holds the PS4's firmware can also cause pulsing blue light of death, and is also catch-22, we cant get the firmware off the chip and onto a new one if the chip is damaged or its firmware is badly corrupted to the point that its not usable after extracting it.
You will find many youtube videos and web sites or forums and even some repair sites stating they fix Pulsing B.L.O.D on PS4's, But the fact is,,, the heat from repair process brings a failed chip back to life for a short period of time, and that's all the chip will ever be capable off..