Can I work in other EU countries with the bluecard I got when I came to Germany?
No, you need to apply for a residence permit from the country in which you would be living and working. You cannot settle in another EU country under the cover of your German residence permit. This residence permit could be a new blue card or a different kind of residence permit, the principle remains the same.
Compared to other residence permits, the blue card does however make the transition easier, under some conditions. In particular, since you have held an EU blue card for more than 18 months, you should be allowed to move to another country and start working while a blue card application in your new country of residence is pending (as opposed to being forced to wait for a visa in your old country of residence).
What happens to my pension contributions I made while working in Germany?
There is a system to consolidate pensions earned across EU countries (see Pension in the EU when having worked in multiple European countries). In Germany, there is also a way to get a lump sum back under some conditions but this only applies if you move out of the EU entirely.
What happens to my health insurance contributions I made while working in Germany?
Nothing. You paid for coverage while you were living there and that's what you got. Unlike pension contributions, which are meant to open up rights in the future (deferred income), there isn't anything to recover with health insurance (or any other kind of insurance, mandatory or not).
Will I lose my bluecard if I accept a job in another EU country?
You will indeed have to relinquish your German EU blue card. You will not lose it because you accept a job in another EU country as such but you will (eventually) lose it if you move out of Germany or leave your current job so the result is essentially the same.
As noted above, you will also need some form of residence permit in your new country of residence. If your new residence permit is also a blue card or if you stay long enough in one EU country to become an EU long-term resident, coming back to Germany should be slightly easier. Otherwise, you would have to start from scratch.
Is there a way I can use/move my pension & health insurance contribution to another country or can I use these contributions that I already made, in another EU country?
Moving health insurance contributions doesn't really make sense, you will need to get coverage and pay for it in your new country of residence (certainly in the Netherlands, where it is mandatory).
In the EU, pensions typically cannot be “moved” per se but only consolidated when you reach retirement age (with the caveat that there are many different systems and not everything is covered by EU rules).
