Knapweeds come into flower in June. Common Knapweed (Centaurea nigra) is indeed common, also known as Black Knapweed and Lesser Knapweed. Greater Knapweed (Centaurea scabiosa) is also widespread. 

 Common Knapweed as seen above has a more compact flowerhead and alternate slender leaves. Greater Knapweed has an altogether bigger flowerhead; the alternating leaves are segmented. Both have vivid purple flowers much visited by bees and butterflies.