Poppy seeds tough after a long simmer
There are actually three types of poppyseeds, the “blue”, the “grey” and the “white” kind. 


The “blue” is the more common one in Europe, it’s dark blueish-grey as grains and produces an almost black mass if prepared as described below. It has a robust, assertive, earthy flavor that is used both in sweet and savory dishes.
The “grey” is a local specialty of the Austrian Waldviertel, and is milder than the blue type, often used in desserts. 
The nutty and mild “white” is a staple in Indian cuisine where it’s used ground and used as binder like flour or nuts.


From your comments I can conclude that you were aiming for a poppyseed pastry filling which is common on the Balkans and other European regions. For this, ground or rather crushed seeds are cooked with a little liquid, often milk, and sugar to create a more or less thick paste. With ground seeds, a short boil and steeping is sufficient. Some preparations skip boiling the poppyseeds entirely and just pour hot milk over them and let them soak up the liquid.

In any case, you need to remember that poppy seeds contain a lot of oil, so many regular mills can’t handle them properly without creating an oily mess. You can also buy the seeds pre-ground, but you shouldn’t store them too long because of the aforementioned oils, which can get rancid quickly.