Nigella is a self sowing annual on my allotment. Excess plants are easy to pull and it doesn't interfere with other plants. It is a feathery thing which doesn't overwhelm. It never leaves the garden area so it isn't invasive.

the foliage is so misty ... even when there is a bazillion of them.. that sunlight will reach things down inside of the mass ... so its probably not an issue ...

and i doubt its roots are so deep.. that they would interfere with a bulb planted 6 inches down ... and the spring bulbs.. will bloom and their leaves brown ... long before the LM are big enough to harm that process ..

i had it for a few years .. about a decade ago ... started taking over 100 sq feet ... i am still killing the errant seedling .. i really hate this plant.. no matter how dramatic a common name ... i just sit here.. shaking my head.. that you are encouraging it .. lol .. but all the power to ya .. that is why its called.. YOUR garden ...

ken

Lol Ken. It's a small area that is completely surrounded- by house walls on two sides and concrete on the other two sides. So it can be as invasive as it wants in this little area well away from my other gardens.